The credit system and salary system have been updated. Nowadays, credits are mainly earned by getting high xp, low repair costs and staying alive. Playing the objective is still a good idea, but so is everything else that will earn high xp. You get paid for being active in combat, which means you earn less if you die often. Unmodded weapons or lightly modded weapons costs less to repair. Tank-hunting is no longer a gold mine and base salary has been nerfed. Other than that, this video still contains some good tips.
The current system added with the Xylander Build (1-8-2015) is that you get paid for playtime. Each rank has amount of salary it gets for 60 minutes of being out in the field. Also, if you stay until the game ends, you will recieve a 10% bonus to this salary. As result amount of credits is calculated using this formula:
Salary * play time * end bonus * veteran, where
Salary – amount of money you get according to your rank in 1 hour, play time – hours spent alive, end bonus – 1.1 modifier if you stayed until the match ended, veteran – having a veteran membership will grant you 25% bonus, meaning 1.25 modifier.
If we take rank 17 for example, it’s hourly salary is 18500 credits. If you were alive for 30 minutes, and haven’t left until the end, and you have veteran membership, your salary will be
18500 * 0.5 * 1.1 * 1.25 = 12718 credits.