Are you knitting continental (German) or throw (English)? Continental will make for a more even guage, but if you're already knitting throw, then you probably just want to stick with that for now. And as the others here said, it's really just practise, practise, practise. Also, depending on the yarn you're using, different needles may help. If it is a yarn with more "grab", such as a lopi wool, you will want more slippery needles, such as the teflon coated metal. But if you are using a slippery yarn, such as silk, then you will want a rougher needle, such as bamboo. That affects how the yarn moves over the needle which will in turn affect the even-ness of your tension.