Take a look at those links Doreen provided, they're great. Also, check out Fern's post about A.M. Cardio on an empty stomach for info on when to do that running. Obviously the morning on an empty stomach is best but if you cant right after working the muscles is good too (as you've depleted glycogen stores).

I've found that by limiting my workouts to every 3 days I can focus much more on what I'm doing when I do do it. With crunches as with all exercises form is more important than quantity. I was going my crunches way too fast and not feeling it, by focusing on the muscles, tightening them and then doing the exercise I found a huge difference. Rest between sets for as much as 2 minutes. I typically do my set of crunches, rest then do a set of reverse crunches, then rest, then do a second set of crunches. It will take longer than 45 seconds this way and you will definitely feel it

HTH
Nat