A journey to Nepal and Tibet can be the experience of a lifetime or a nightmare. Often it is both! The vast majority of Westerners come away from these pilgrimages thankful that they went and admitting to personal and/or spiritual growth that is impossible to value. 
        There are many Tibet "packages" out there and it is not always easy to comparison shop.  I have composed a check list of some things that I feel are important considerations. You will be asked to send a deposit and sign a contract when you decide on a trip.  The contract determines what you will get and what you will owe.  Read it carefully.  If it's a good agent, the contract will be quite specific. Hopefully this list of questions will be helpful in your search for the Tibet pilgrimage that will best for your needs:

Price means nothing unless you know what you are paying for.  Things to consider  include:

Flights to Tibet:   What flights are included?  If there is a flight from a more Eastern Chinese City to Lhasa, Tibet, or from Kathmandu to Lhasa, is that short flight included in the fee. Internal flights like this can add $500 or more USD to the price.   Usually, the International flight is not included but smaller flights are.  THIS IS NOT ALWAYS TRUE.  If the price states "Land Only"  find out HOW MANY FLIGHTS YOU WILL NEED TO PURCHASE.

The Guide:  Without a qualified guide the trip can be a total waste and perhaps even dangerous.  Ask if the guide has spent considerable time in Tibet, if he or she speaks the languages, knows first aid, has proven leadership skills and the appropriate personality for the job.  How many trips has the guide led, and for what groups?  Is the guide knowledgeable about the culture, art, religion and people?  Can you speak to someone who is familiar with him or her?  What information is available about the person? "Google" him or her!
        If the trip leader is a religious figure or translator, check to be sure he or she is also trained as a guide and has actually spent considerable time in the country. Sometimes religious leaders are limited as far as what parts of Tibet they can visit and often they are not allowed to visit Lhasa.  If you have never been to Tibet before, however,  you must see Lhasa since all the great institutions and art are in that area so be sure your leader can travel there.
Often trips are organized using only local guides.  But In China/Tibet you will get a lot more from an additional knowledgeable Western guide. One more consideration:  Are you taking this trip because it is led by a particular guide?  Make sure you can cancel if that guide is not available for some reason.
        I am constrained to mention another issue here:  A few guides have been known to be so inflexible, opinionated and humorless that they can make the trip a real ordeal for the person who is not of their religious persuasion.  Do yourself a favor and do some investigation. Most travelers do not want to be scolded, demeaned, or told what to believe.  This scenario is the exception but be aware and if you are not sure, talk to people who have been on one of their trips.

The Hotels: What hotels will be used?  For how many nights? Hotel prices are usually the single most important factor in determining a trip's price. If you start in Kathmandu,  Is the hotel first class? If not, does it have private baths and at least a nice courtyard?  Is it  centrally located so you can walk out by yourself and explore?  Are the hotel restaurants and rooms nice?  Is there a pool, health club, terrace, grassy area, etc.?   So many programs do not even disclose the name of the hotel they will use.  Most good hotels in Nepal have a web site and are mentioned in the guidebooks. Because of the political climate, some will feel better in a first class Kathmandu hotel.  My choice is the Yak and Yeti.  If you prefer to really be on your own separate compound, close to the airport,  the Hyatt wins. Both are wonderful.
      In Tibet, hotel considerations are a bit different. No hotels are what Westerners would call "first class" but comfort, safety, warm water and good food are the serious considerations.  The ideal location is near the Jokhang Temple, the center of the Tibetan section of the city.  My choice, hands down is the Keichu Hotel (Tibetan family run with great food and wonderful antique shop).  Here, it is easy to walk out and explore, shop, etc. and meet people.  The Barkor Market is close and many people spend hours here interacting with merchants and shopping for all kinds of exotic and beautiful dharma items.

The Itinerary:  Will you enter Tibet via China (usually flying in from Chendu, possibly going overland or riding the new train), via Nepal or on some other route.  My own favorite itinerary begins with a few days in Kathmandu.  Why?  Because Kathmandu has great restaurants and hotels, Dharma Centers, history, and the people are wonderful.  Also,  acclimatization to the 12,000 foot altitude of Lhasa is a lot easier after spending some time at 4500 feet in Nepal.  Due to the Maoist problems in recent years, many agents have bypassed Nepal and going into Tibet through China.  This has the disadvantage of going to Tibet's high altitude directly from sea level.  Also, Nepal is not to be missed.  Tourists have been safe in Kathmandu and problems there are abating.  Nepal is the birthplace of the Buddha, has some of the nicest people you will ever meet and is home of many of the world's great spiritual teachers, writers, artists, etc.  It's a fun place to shop for antiques to trinkets, and has a lot of history.
       I would have absolutely no qualms about going to Kathmandu for a few days,  staying in a first class, or any of the other nice hotels, and then flying across Mt. Everest and the Everest range to get to Lhasa.  In fact, as of this update (2007 fall) most agents are resuming travel and the situation looks a lot better that it has in recent years.  But labor strikes and other inconveniences  that occur in Nepal can be inconvenient. AS for me, for group pilgrimages, I I prefer the Kathmandu route.
       How many days are included?  An agent can cut its cost of a trip considerable by making it a few days shorter.  Will a rushed itinerary create an acclimatization problem?   Will you have a chance to see the things you want to see?  Is there flexibility? Can you stay longer? 
        Will you fly in and out of Tibet?   If you are going form Nepal to Tibet is there overland travel one way?  Overland travel out of Tibet sounds good but in reality it can be a very bumpy, dusty and long bus ride with marginal hotels and food along the way.  Areas like Gyantse and Shigatze can be just as easily reached by land cruisers out of Lhasa in a "fly in--fly out" itinerary.  The Lhasa--Kathmandu flight is expensive and it is cheaper to drive out than to fly out. This is why some agents select this plan. 
         Give some thought to altitude: Is the itinerary planned so that the altitude starts lower and climbs as the trip proceeds?  How high does the itinerary take you and when you are at the higher altitudes will you be driving or walking?   It's one thing to drive over a high pass in a jeep and quite another to hike it.   Where will you be? If high altitude days are included can you descend fast if you become sick? 

City vs. Outlying Areas:  I just heard the author of Lonely Planet Tibet  interviewed on NPR. The thrust of his message is that you need to get out of Lhasa to see a Tibet that is less changed by Chinese influence and population influx.  I agree.  Still,  if you have not been to Tibet, there is much to see in Lhasa that you would not want to miss.  You also need to spend some time at Lhasa's (relatively) lower altitude before going higher.  Lhasa has become a regular Chinese tourist destination and the old culture is slipping away fast.  (Another good reason to stop in Nepal where so many traditions are alive and practiced more freely.)

Meals: How many are included and how good will they be?  Breakfast is usually at the hotel so the quality of the hotel always affects the quality of the breakfast.   Are there lots of  especially nice restaurants included or just one upscale "farewell dinner?"  Lhasa food is pretty good to excellent but when you leave Lhasa food can be a problem for many (especially vegetarians). Continued ...