I had to set up rules for myself.
For SS 12", the main part of my collection, I limit myself to one version of a character. (That's going to be hard when Luke X-Wing pilot goes on sale.)
For other lines, such as the GG minibusts, I only buy them if they're ones I can't live without, such as Chewie and Mara Jade. If they're at all mediocre, like Qui-Gon, I pass.
For other higher-priced lines, such as the GG animated statues, I only allow myself one purchase from the line. (I went with Leia & R2.) Only one LE saber from MR, even though I want a bunch of them.
And I collect one character, Obi-Wan, meaning all bets are off and the rules don't apply. I can pretty much get anything for that character. Multiple 12", PF, minibusts, nesting dolls, Galactic Heroes.
It's tough, since I have to pass on so many great items, like the animated Boba statue, the SS 12" Boushh and a bunch of other things, but there's only so much money and display space for this stuff.