I also use google translate and also rate it as shit.
As well as avoiding slang, abbreviations etc. I think it helps to keep sentences as short and simple as you can, no commas and so on.
Also, once you've translated into Russian you can try translating back into English (or your language) and if the result is gobbledygook rephrase stuff and try again. Kind of time wastey though, if any one knows a better translater that would be mega!