There's an edging knife thing (like a little spade with a flat, sharp edge) you can get in any DIY/Garden shop. They were in Tesco last year for about a tenner.
Or my way; get your taut line of twine and make sure it's straight, then score along the line with a spade with a straight edge. Remove the twine, then with the back edge of the spade away from you, cut into the lawn (only just as far as you have to) and voila.
I think you can use a strimmer to keep it straight but I don't bother. Just get my spade out every so often.
Rebecca