Eating lots of clean foods. I have been doing a 500-1000 calorie surplus and I'm not really gaining body fat, mainly muscle. Everyone is different though and some people react differently. Personally I'm not a fan of the not worry about body fat gain method as that is very much a new concept. I got a chance to talk to Lee Haney recently and he says it makes him sad to see people buying into that method. He said the best measurement to tell if you're eating too much is to look at your abs, if you can't see them very well then lower your calories. At the same time though you don't want to be too shredded while trying to gain size because it is counter productive.