I heard that the reason cheetah populations are so small is partly because of their bad sperm and because their all so closely related. So i was wondering, Since they're all so closely related, why dont humans try to cross breed them with another similar cat species such as leopards?

because that would end up with hybrids that are sterile and cant breed and leopards are the one of the great cats and cheetahs are not

Not all hybrids are sterile. Many people think that they are, and thats how we end up with Ti-Ligers or those such. Many of the males are, but some females are and if we introduce them into the wild, they could breed and in doing so, bring new genetics to the cheetah species. it'd be better then them all dieing out, right?

http://www.lairweb.org.nz/tiger/tigons.html

if they were crossbred with leopards, it would likely only further damage their genetic make up.

but isnt there some way we can save a part of thier species before they all die out? I'd hate to see them go out like the thylacines did.

unfortunately no, a depleted gene pool can not be repaired by human intervention, so it can only be corrected by nature which may take hundreds or maybe even thousands of generations.

but, of course, that doesn't mean cheetahs are doomed to extinction. the northern elephant seal population was once down to only 8 individuals, now they number well over 100,000 despite having low genetic diversity. so ther is a chance the cheetah can recover.