Economy (EU2012)

Only buy or build what you "need" rather than what you want.

For example, you want to outfit every Soldier with a Nano-Fiber Vest early in the game. But, you also have to carry some grenades, a medikit, maybe a SCOPE or Arc thrower, and it will be awhile before your squad size hits 6. Think of it this way, you could spend 20 credits for a single vest that provides 2HP, or you could spend 20 credits on two complete soldiers who might start as Squaddies (if you bought that in the OTS) and might have psionics. Manufacture enough of each item that you think you'll actually equip, no more. If you find yourself equipping useless things, then go back to Engineering and buy some more of something useful. Save your money upfront, because most of the stuff you build in Engineering cannot be sold on the Gray Market and you never know when a country will request something or how much. Cost avoidance especially applies to rare materials like Elerium and Alloys; once they're spent, they are gone.
Another exmaple, take the time to un-equip items and armor from troops to re-equip them on whoever is going out on the mission. It may take time, but it will save resources in the long-run. Note that injured soldiers who go to the infirmary automatically un-equip all their special gear. Soldiers in Psi training keep their equipment, you have to un-equip it manually or you won't have access to it for ten days.

When stuck on a decision, choose what you think will impact the game the most. No duh, right?

For example, you could skip the OTS project to make soldiers recover from injury faster. This could actually help force yourself to work in a rotation and keep some Squaddies in reserve for those times where your guy dies rather than just getting injured. Conversely, you can save those credits for the eventual OTS project to make higher rank soldiers more likely to survive critical hits the higher their rank. Think of it this way: soldiers are going to get hit. Period. Give them a project that helps them survive and take their recovery time as an opportunity to build up some reserves.