This is something I have written about before, but it bears repeating. When you live a generous life (whether it be with money or time), God allows you to do more with what is left. The urge is always to cut back on charitable giving when money gets painfully tight, but I have found that those are the times I actually need to give more (at least percentagewise). Sadly, I have yet to make it to the Biblical 10 %, but I keep aiming for it.
For more evidence of this phenomenon, this article by Marilynn Judd offers much to think about: Adventures in Tithing