I've been reading up on Lith ion a bit. There is no memory. But there is a chip in it to regulate the charge. Try running the battery down all the way a few times then charging it fully. This will help recalibrate the chip. Then, going forward, give it many tweeking charges as opposed to what I just described. Might help.
Heat is the enemy! Don't leave 'em in a hot car and when they start to get hot with use try to slow down. I used to take the back off my Epic and massage the battery (while installed and running) to let some of that heat dissapate into my hand. Sounds weird but it can help. But when it's hot to the touch , that's murder on the battery.
Remember, Google is your friend! In this case anyway.
also if your battery life issues started after the last update it might be that as well. go into your running apps in your settings area. see what all is running. might as well go into about phone and hit battery and then battery use. it will show if anything is running higher than it should be.
Click on this link and it will give you some tips to conserve the battery life on your HTC EVO 4G: http://www.sprintdroids.com/forum/htc-evo-4g/112-20-tips-improve-battery-life.html.
I hope this was helpful.