i'm not sure if you strain your neck for the same reasons i do, but my neck always hurts because of all of the ab work. i really don't know how to do ab work without tensing my neck. i adjust a lot of the moves and use my hands to support my head while i'm doing them (or have a hand towel under my neck so it's still slightly elevated but relaxed). the form isn't perfect, but until my abs get strong enough that i no longer need to stress my neck, it gets the job done.
I used to take a yoga/pilates class - I found that I just have to be very conscious of how I'm holding my body. I naturally tense my shoulders and neck, so I'd always have to tell myself to LOOSEN UP. But then after a while, I got used to it and just had to spot check myself.