Just enjoy the game story and leveling process rather than getting bogged down with making lots of credits
I'll never understand people who fuss over making credits before reaching maximum level. Just doing your average leveling missions should give you more than enough credits to buy skills with (and with credits to spare if you're the type who likes to keep the mods in their gear up to date). Just sit back and enjoy the game while leveling and, if you avoid making silly purchaces on the GTN, you should have no problems with credits.
Once you hit 55 (or 50 if you don't have the expansion) you can just do dailies for credits and be surprised by how many you make.
To give an example, I haven't focused on making large amounts of credits but just by doing weeklies every now and again I've made around 5 million over the past couple of months, even after making some silly purchaces on the GTN.