Tension can be a tricky thing to master. This is why a lot of patterns will include a guage to indicate how big a certain number of stitches should be.
I tend to wrap the yarn throughy fingers on my off hand to help keepy tension consistent. I'm sure there are other tricks you could try too. Maybe someone else will have a suggestion?
I gotta get that. I tried knitting first and spent months being frustrated that I could get anything to look how I intended. Tried crochet and felt like I had been doing it my whole life.