Using a band on my ankles always helped me keep my legs straight. It was a really thick rubber one, I think the coach cut up a tyre to make it, but it did really help.
Lots of perfect leg ups also helped me, a really fun drill was to put a squishy ball about the size of a tennis ball just above your toes and/or inbetween your knees, if you dropped them you got another ten reps as well as restarting the ones you had already done.