Preserve me, O God - Keep me; guard me; save me. This language implies that there was imminent danger of some kind - perhaps, as the subsequent part of the psalm would seem to indicate, danger of death. See Psalms 16:8-10. The idea here is, that God was able to preserve him from the impending danger, and that he might hope he would do it.
For in thee do I put my trust - That is, my hope is in thee. He had no other reliance than God; but he had confidence in him - he felt assured that there was safety there.