Getting 5 stars is based on points. Try to get the most points you possibly can in a mission, and understand what gets bonuses for which missions. Deadlines, for example, award you bonuses for getting through the mission with a lot of time remaining. Elite gives you bonus for not taking damage. Covert gives you bonuses for not killing enemies. Other than Covert, there's a bonus for killing all the enemies in a mission, so make sure you get everyone in a mission when possible. Usually, when you finish an objective, the game spawns enemies in the surrounding area, so keep an eye on that.
Here's some scores to shoot for (thanks to Sherufanir and ps3trophies.org for the list):
1: 95,182 (you can go lower)
2: 68,418
3: 95,878
4: 104,757
5: 87,195
6: 130,967
7: 79,227
8: 125,393 (you can get lower)
9: 97,957
10: 111,196 (can get lower)
11: 105,372
12: 99,498
13: 87,465
14: 124,856
15: 85,950
16: 101,109
17: 134,010
18: 86,804
19: 78,143
20: 104,694
21: 102,150
22: 87,853
23: 155,348 (can get lower)
24: 100,792
25: 79,110
26: 105,856
27: 232,292 (can go MUCH lower)
28: 92,791
29: 99,428
30: 130,623
31: 111,886
32: 89,650
33: 157,372
34: 152,195
35: 120,700 (can go lower)
36: 276,675