My tree is about 10 years old, it has six main trunks and is roughly 20' tall. The trunks don't seem to get any thicker and Now that it has bloomed, the flowers have pulled the tops to the ground. What should I do?
This is common with Crepe Myrtles, and some other shrubs. With most plants, there's an obvious feeling of "large branches = healthy/sturdy plant", but with CMs, there's too much flowering and too much foliage on larger branches. The foliage/flower weight increases at a rate greater than what its branches can support.
Crepe Myrtles require yearly pruning in the end of winter.
For the rest of this year, deal with the droop, and tend any snapped branches (go in with a trimmer/saw and lop off the snapped branch close to its parent).
Next late winter / early spring (before dormancy ends), get in there with some heavy pruning.
This seems like a good guide: http://thedailysouth.southernliving.com/2009/02/24/what-concerns-p/
I found that article a while back, but that's how I've always done any tree or shrub. I start with dead branches, then suckers, then I limb it up to whatever height I want it.
Even shrubs, because you need them high enough to mulch or string trim under. Then I go to crossing branches, removing the whichever one looks like it will conflict with healthy growth the most. Any branches growing to the inside pretty much have to go. If it's a shrub or tree that has multiple trunks, just pick ones that won't rub as they get larger in diameter.
Keep in mind while doing this that you shouldn't take more than 1/3rd of the plant off at one time for most plants (not all of them though, some you can cut to the ground and they're fine). Also, you need to try and imagine where the tree is going. Look at small branches and see that two will eventually cross and rub as they get larger. Save the tree the energy of growing one that you'll lop off later and take it when it's small. It's also important not to force a tree to try to grow to fit your expectations. I let it grow however it wants and remove material only to keep it healthy. If you have to turn it into a giant bonsai, then something is seriously wrong.
Lastly, keeping all of that in mind, sometimes something just has to go. For instance, we have one crepe myrtle that was planted from a 3gal pot. I've been pruning it this way one a year for 7-8 years now. It's currently about 9' tall, has several trunks and has just started to get the smooth pretty bark in the last year or two. I went to prune it this year and noticed that while 4 of the trunks had really nice growth patterns, one of them was leaning way out. It's just as tall and thick as the others, but it's just turned into a really horrible looking trunk, leaning way out to the side. It'll get worse as it gets older. I let one well positioned sucker grow off of this "trunk" and because of the fertilizer and good root system, it's now almost 5' tall and thumb thick just from a half summers growth. Next spring I'll just have to remove the one trunk and let the sucker replace it.