Cheap Cosmonauts
On the Soviet moon mission, you can buy cheap cosmonauts by taking advantage of a little mathematical error in the game. Cosmonauts cost $600, but you get a free one when you sell barracks, which cost $500. You can essentially build extra barracks to sell instead of paying more for cosmonauts.
Free Brutes
You need to play as Yuri and make sure the "Superweapons" option is turned on. First, build a genetic mutator, and wait for it to be ready. While this is loading, build a group of slave miners and send them into one spot. Use the genetic mutator to turn your slaves into brutes.
Remove Trees
If trees are getting in the way of your progress, you can destroy them using a grizzly, rhino, or laser tank. This gives you more room to expand and create a stronger base. The trees take about five shots to destroy.