For flop -> turn or turn -> river, I learned (Outs x 2 / 100 - 1). That gives you the ratio your draw will hit, which is my preferred method rather than percentages. For instance with a flush draw, you have 9 outs. So 9 outs x 2 = 18... divided by 100 is about 5, then minus 1 = 4. So your odds are about 4:1 of hitting your flush draw on the next card.
If you're put all-in on the flop and can see the next two cards without any further betting, then you slightly revise the formula to (Outs x 4 / 100 - 1). Now your odds of hitting the flush with two cards to come is 2:1 (9 x 4 = 36 / 100 = ~3 - 1 = 2).
I found a webpage once that went into thorough detail on many probabilities involved in poker. Actually, it was mentioned right in this forum... just found it:
http://www.math.sfu.ca/~alspach/comp35/.
With regard to overpairs versus underpairs, taken from the table at the bottom of the page (I slightly modified it for convenience):
4% KK v AA
8% QQ v KK+ (read KK or AA)
12% JJ v QQ+ (QQ or KK or AA)
15% TT v JJ+ (same idea)
18% 99 v TT+
21% 88 v 99+
23% 77 "
26% 66 "
28% 55 "
30% 44 "
31% 33 "
32% 22 "