Aim high if you're still leveling
If you're level four, for example, you're going to get fewer benefits from an Adventure Pack such as The Catacombs (level 3-4) than from something like Three-Barrel-Cove (level 4-7). If you purchase an Adventure Pack that begins at your current level -- or even a level higher if you're feeling brave and have some good friends to run it with -- you're going to get a lot more experience out of it, and it will be useful to you for a much longer time.
Of course, I'm certainly not recommending you purchase an Adventure Pack that sounds terrible to you solely because it's a level higher than where you currently are, but that is definitely where you'll want to start your search. You'll find quite a few options in your level range, so the opportunity to find something that sounds fun is there. The ability to set the difficulty each time you enter a quest will allow you to change the content as you level on each character, so the Pack will keep up with you to a certain point. Keep in mind also that the Pack will be unlocked account wide, so any character on any server can use it, not just the character you bought it on. Take a peek at your alts, see what you've got on other servers, and take all of your characters into consideration. That should give you a good starting point to begin shopping.
Take advantage of sales
Turbine is great about sales and promotions, and you can pretty much always find an item temporarily marked 20-50% off in the store. Sometimes they are smaller items -- the current sale, for example, is 50% off a package of Potions of Jumping -- but many times you'll find 20% off of selected Adventure Packs. If it's a Pack that interests you, it's the perfect time to spend some of those Points you've been setting aside for a rainy day. It's pretty easy to keep up with the sales via DDO's Store information page on their site, or by subscribing to their RSS feed.
Think about what you are interested in doing
An Adventure Pack is so much more than a bundle of random quests. There are storylines, different items to be had, different play styles, and much more. If you're interested in named items, for example, you'll naturally want to look at Adventure Packs such as Delera's Tomb for the Voice of the Master, a little trinket that gives you a permanent +5% XP boost.
If you're an explorer at heart, quite a few of the Packs feature wilderness areas. These offer slightly fewer quests, but instead give some gorgeous explorables with hefty XP rewards for slaying the hordes of creatures running around them, as well as rewards for killing bosses and finding landmarks. If you prefer to take your time, do some wandering, and see the sights, look at the Adventure Packs that mention "wilderness area objectives."
Even within the Adventure Pack quests, you'll find a wide variety of gameplay from hack-n-slash fighting to puzzle solving. Most people prefer one sort of questing over another -- I love puzzles, for example, but know quite a few players who can't stand them -- so take a look around the Quests & Adventures forums to get a feel for what sort of things the different Adventure Packs offer. If you've got a Rogue and want to spend your time springing traps, opening secret doors, and generally being sneaky, search the forum for a Pack that fits well with a stealth character. The strength of DDO's Adventure Packs lies in large part in the variety, so poke around a bit and take advantage of that to find something suited to your play style.
Take advantage of guest passes if possible
One of the best things about the DDO
Adventure Packs is the ability to check them out through Guest Passes. Guest Passes allow those players who do not own an Adventure Pack to access it for a limited amount of time as the guest of someone who does own it. Passes can be purchased in time increments from 30 to 180 minutes, for as little as 15 Turbine Points. If you've got a pack you're interested in but aren't quite sure, put out the call with your guild, friends, or even the forums and ask if someone will guest you so you can have a little preview. Don't be greedy, of course -- stick with the 30 minute pass, that's plenty of time to get an idea of the content.
Once you've got the guest pass, it's best to take a brief look at the content of each quest as well as wilderness areas if they apply. Give yourself a brief overview of everything and see how you feel about the Pack as a whole.Now go shopping!
In the end, there are so many ways to get the most use out of your Adventure Pack purchase that it's a matter of picking your highest priority and moving on from there. Do you want to level? Explore? Earn favor? Hopefully this will help you start narrowing down what you'd like to pick up.