I did yoga (a fairly easy variety) for a few years. Then I decided to go back to school for a post-grad degree while working full time. Result = not enough time to do yoga (or much of anything else) . . . and a lot of stress.
When I finished school and started doing yoga again, I felt as weak as a kitten. I started doing power yoga. My initial goal was just to gain enough strength to make plank and downward facing dog comfortable.
After that, I decided it would be cool to be able to do wheel pose.
I accomplished it fairly quickly, but now I tend not to do wheel pose too often. Maybe a couple of times a month. It bothers my lower back a bit if I do it too frequently.
Next, I focused on arm balances such as crow pose and side crow. I can do them (barely) but I have a balance issue. I tend to fall over. Once while doing side crow, I fell right on my face. It didn't hurt because I wasn't very far off the ground, but it was a bit embarrassing.
My current goal is to be able to do a headstand. I have gotten part way there. The first step was to be able to do forward straddle fold and while being able to put my head on the ground.
I reached that step March or April 2012. Next step after that was to get into a tripod headstand without legs extended.
I have been able to do that for a few months now. But I found that I don't have enough core strength to extend my legs in the air.
My New Years resolution is to build enough core strength to accomplish this goal. There are a lot of crunches in my future.