How do you get rank:
The only way to get rank is to bomb and take as many planets as possible in a short amount of time. The problem most players have is they do not take enough planets per hour. Every time you bomb and take a planet your DI will go up. But you also need Stats (rating)

Rank is figured on three things. The amount of DI you have, your Rating (stats), and the amount of time you have spent playing. Your DI and Rating (stats) will only go up / down while in T-mode.

Promotion also occurs at:
2xDI with rating -1
4xDI with rating -2

* Once you reach to rank of commander you can command a star base.

How does the rating/promotion system work?
Do a shift-I on yourself; you'll see four ratings there, one for planets, one for bombing, one for offense, and one for defense. The planet, bombing, and offense ratings are measured in DI/hour; for each planet you take, army you bomb, and player you kill you get a certain amount of DI (which varies depending on the server)--the average amount per hour is your rating. Your rating is equal to (your DI/your hours)/ (total DI/total hours); therefore, if you have a bombing rating of 2.0, you bomb twice as many armies as the average person on the server. This is true for all ratings. All ratings are updated only in t-mode. Now do a shift-U. Listed there are the thresholds for each rank. Your overall rating is your planets+bombing+offense. To make a rank, you need to have the DI required for that rank, and your ratings need to be above the rating required for that rank (i.e., to make Admiral, you need 320 DI and your ratings need to be above 8.0). You can also make a ranking by having ratings good enough for a level and twice the DI you would need for the next level (So you can make Admiral with ratings of 7.0 and 640 DI). This equation is recursive, so you can also make it with 6.0 ratings and 1280 DI (which would take about 200 hours), or 5.0 ratings and 2560 DI (which would take 500). On some servers the equation is recursive only once, so you can't make ranks on 8xDI.

On some servers, your defense also needs to be above .8 to advance past Lt. Commander.