If you want realism there's no one simple system. A linear system where X number of points over the course of a campaign nets you Y medal doesn't work. Medals could be awarded months or years after the fact and what medal you got depended on a number of factors, from politics to who saw you do the deed.

The simplest realism fix for a "point based" medals system like that which exists in IL2 is to just say that "you've been recommended for"; [X]. That covers everything from Japan (where decorations generally weren't given to living airmen) to the U.S. where low-level decorations were routinely awarded for certain actions (i.e., the 8th AF generally handed out an air medal for each aerial victory and every 25-50 bombing sorties).

If you want something a more realistic, consider 3 systems:

Linear: Like the Nazi German system, where you were awarded successively higher ranks of the Knight's Cross for cumulative success over time. This is identical to the current IL2 system.

"Threshold-based": The game tracks mission performance and overall performance, factoring in things like the odds against you, whether you were wounded, and whether you were flying over friendly territory. If you hit a certain "threshold" you are recommended for a certain type of decoration, with some percentage chance that you'll actually get it at a later date. You can get subsequent awards of the same medal, and you might go your entire career without ever winning the higher level decorations, even if you shoot down a huge number of planes. This is realistic for the US, UK, USSR Italy and most other nations.

Nothing: No awards. At most, you get a recommendation for an award using the threshold-based or linear systems. This is realistic for Japan for much of WW2, as well as for pilots flying for the AVG (although they got $500 per plane shot down, which is its own reward, and they were later given decorations by the Chinese Nationalist government).

Try the EAF51.

http://www.europeanaf.org/forum/forumdisplay.php?f=7