When doing full cross stitch over one on evenweave, I always thought I could just stitch as usual as long as I finished each cross as I went -- I knew not to use my usual "row of half crosses, than back and cross them all" method because half cross stitch slips under the weave.
But I did still find that some of my small crosses went a bit wonky and seemed to disappear; not very visible on 28ct over one, and hardly worth noting on 36ct over one, but still annoying.
Then I read something in New Stitches which made a lot of sense (I should have thought of that earlier, really

The solution?
When you've done the first half of the cross stitch, make sure that when you come up for the second half, the thread on the back lies parallel with the top thread of the weave. It takes a bit of thinking at first, but it soon becomes automatic -- I found that doing my Time For God bookmark last night!
By the way, I just re-read that last paragraph and it's not very clear -- I'll try and do a diagram. The top two stitches are if you make your first half from bottom to top, the bottom to stitches are if you make your first half top to bottom; the dotted line shows the thread going round the back to the hole starting the second half of the cross stitch:
