Tip 1, try and find a more level pitch! (Yes, I know it can be difficult.)
Tip 2, go to Wilko or a hardware store and buy a roll of antislip matting.
It's like a sort of black rubbery-feeling mesh, the one I used until recently is 100cm x 30 cm (40 in x 12 in) and weighs 96g (3.4 oz). Rolled up it takes no space in your pack. I used to put it under my self-inflating ground mat and it was very effective at stopping it sliding around the tent.
Tip 3: Not really recommended. Last year I bought a can of spray-on antislip designed to be applied to the underside of rugs etc. I thought it would put a rubbery surface under my groundmat, but it has remained sticky, and while it does the job really well of stopping the groundmat moving, it has resulted in little bits of dead grass and heather stuck over it, also when I fold the mat down the centre to roll it up the two halves stick to each other.