Ok, he called your preflop bet and the flop's a mess.  Now what?  Remember, the average winning hand in hold 'em is two pair.  Be careful with TPTK(Top Pair, Top Kicker), but if you're holding two pair, chances are you're in pretty good shape.  Below are the headsup win/loss precentages for a variety of post-flop situations.

 
Overcards vs pair
Board 5h 2c 8d
win % 
tie %
odds
As Kd 26% 0%
2.9
 to 1
Th Tc 74% 0%
0.3
 to 1
Overcards vs open-ended straight draw
Board 7c 8d 9s
win % 
tie %
odds
As Kd 51% 2%
1.0
 to 1
2h Tc 47% 2%
1.1
 to 1
Overcards vs flush draw
Board 7c 5c 2c
win % 
tie %
odds
As Kd 47% 0%
1.1
 to 1
Tc 4h 53% 0%
0.9
 to 1
Overcards vs open-ended straight, flush draw
Board 7c 8c 9c
win % 
tie %
odds
As Kd 32% 2%
2.1
 to 1
2h Tc 66% 2%
0.5
 to 1
Overpair vs underpair, open-ended straight draw
Board 7s 8h 9c
win % 
tie %
odds
Ks Kh 61% 2%
0.7
 to 1
Td Tc 38% 2%
1.6
 to 1
Overpair vs underpair, flush draw
Board 2c 5c 9c
win % 
tie %
odds
Ks Kh 58% 0%
0.7
 to 1
Qd Qc 42% 0%
1.4
 to 1
Overpair vs underpair, open-ended straight, flush draw
Board 7c 8c 9c
win % 
tie %
odds
Ks Kh 39% 1%
1.5
 to 1
Td Tc 59% 1%
0.7
 to 1
Overpair vs two pair
Board Kh Jd 6c
win % 
tie %
odds
Ac Ad 27% 1%
2.7
 to 1
Kc Jc 73% 1%
0.4
 to 1
Top pair, top kicker (TPTK) vs two pair
Board Kh Jd 6c
win % 
tie %
odds
Ac Kd 15% 1%
5.9
 to 1
Kc Jc 85% 1%
0.2
 to 1
TPTK vs overpair less than top kicker
Board Qh 6c 2s
win % 
tie %
odds
Ac Qd 20% 0%
4.0
 to 1
Kc Kh 80% 0%
0.3
 to 1
TPTK vs set
Board Kh Jd 3c
win % 
tie %
odds
Ac Kd 4% 0%
27.2
 to 1
3h 3s 96% 0%
0.0
 to 1
TPTK vs flush draw
Board Kh 7d 2d
win % 
tie %
odds
Ac Ks 61% 0%
0.6
 to 1
Td 9d 39% 0%
1.5
 to 1
TPTK vs open-ended straight draw
Board Kh 7d 6h
win % 
tie %
odds
Ac Ks 66% 0%
0.5
 to 1
9d 8c 34% 0%
1.9
 to 1
TPTK vs open-ended straight, flush draw
Board Kd Th 9h
win % 
tie %
odds
As Kc 45% 0%
1.2
 to 1
8h 7h 55% 0%
0.8
 to 1
Two pair vs open-ended straight draw
Board Kh 8c 7d
win % 
tie %
odds
8s 7h 70% 0%
2.3
 to 1
5d 6s 30% 0%
0.4
 to 1
Two pair vs flush draw
Board Jc 8c 7d
win % 
tie %
odds
8s 7h 65% 1%
2.0
 to 1
Ac 5c 34% 1%
0.5
 to 1
Two pair vs open-ended straight, flush draw
Board Kh 8c 7c
win % 
tie %
odds
8s 7h 51% 0%
1.0
 to 1
6c 5c 49% 0%
1.0
 to 1
Two pair vs Two pair
Board Ac Ks 9h
win % 
tie %
odds
As Kh 92% 0%
0.1
 to 1
Kd 9d 8% 0%
11.2
 to 1
Set vs open-ended straight draw
Board Th 9s 3c
win % 
tie %
odds
3s 3h 74% 0%
0.3
 to 1
8c 7d 26% 0%
2.9
 to 1
Set vs flush draw
Board Th 9h 3c
win % 
tie %
odds
3s 3d 74% 0%
0.3
 to 1
Ah 5h 26% 0%
2.9
 to 1
Set vs open-ended straight, flush draw
Board Th 9h 3c
win % 
tie %
odds
3s 3h 58% 0%
0.7
 to 1
8h 7h 42% 0%
1.4
 to 1
Two pair vs set
Board 8c 7d 3h
win % 
tie %
odds
8s 7h 17% 0%
5.0
 to 1