A common English translation of 一応 is "for now", which does work sometimes, but definitely not always.
Let's look at the dictionary entry first:
http://dictionary.goo.ne.jp/leaf/jn2/12328/m0u/
First there's noun which means "one time" or go "one time". I don't see those used too often.
Then the adverbs. The first meaning is "not thoroughly, but generally or roughly done, outlined or approximately done". As in これで一応でき上がりだ to say something is roughly done or outlined.
The second meaning is "just in case". the same as 念のため, as in 一応見直しましょう.
...and that's all the dictionary has. This is probably unsatisfactory though, because it seems like people use it more broadly. What finally made it click for me was
this person's lang-8 post.
There are a lot of good comments on that post. They generally say that people can also use it as an expression of humility. Here's the example from one of them:
A:このケーキおいしいね。
B:ありがとう、一応それ私の手作りなんだ!
That 一応 is probably the one you're looking for (it's the one I didn't fully understand for a long time). The commenter would interpret that 一応 as(いやいや、十分といえない、まだまだです). So A calls the cake delicious, and while B takes credit for making it, he doesn't want to boast about it, so he adds 一応.
Edited: 2013-03-26, 7:56 am