Level 25 Commando here.
I generally try not to spend any money on armor, weapons or stims. I use commendations or whatever I loot for my needs.
Never repair before equipping your best stuff that you just looted. Otherwise, you'll end up repairing the armor and weapons you're about to get rid of to the vendor.
If you loot a blue or purple that you can't use and none of your friends can use it... put it up for sale on the galactic market.
Some people only upgrade abilities that they use each level to save credits for level 25, but I upgraded them all and learned each one.
I crafted some, but not a lot. (Up to about 90 in synthweaving) What materials I could harvest myself; I did rather than send my companions out. (cause that will eventually lead to lots of credit loss)
I did purchase extra space for my immediate inventory -5000.
I really can't think of anything else to conserve credits up to this point in my characters life.
Upon reaching 25 I was able to purchase training to ride my speeder (-40,000) and I still had almost 40k left. Now I have sent my companions out on missions and I'm down to 10k lol