Save your cash. When you first enter the game, you are given a default amount. Unless the server has specifically altered the game settings, the basic amount should be $800. The best strategy for racking up a good size chunk of cash is to not buy any weapons or protection for the first few rounds. It’s far easier to hang back with the standard sidearm, and steal the better weapons off of players who have already bit the dust.
Win the round. Although players are given a standard amount of cash at the beginning and at each respawn, the amounts deposited for the winning team are significantly higher than those of the losers. In addition, if you manage to survive the round, you not only get the money for respawning, but you don’t have to spend anything on weapons or protection, as they carry over from the previous round. In this case, playing well also means that you will play economically.
Bet on the outcome. Many servers have the CS Gambling Add-On. This nifty little utility allows players to wager on who will win the round, and if you are good at it, you can make a ton of cash really fast. You simply type in the commands to the program in the game’s chat bar, and it will automatically place the appropriate wager. If you win, you will receive payment that is double what you put into the pot, and it's available right after the round finishes.
Play well. Again, this is dependant on the particular server that you happen to be playing in, but most if not all servers now-a-days utilize add-on’s and plug-ins that grant cash incentives to players for their performance. This can range from cash bonuses for headshots or multi-kills, to knocking off the player that was doing so well that the other team placed a bounty on his head. The possibilities are limited only by the servers themselves, so happy hunting, and I hope that I’ve helped.