Remember those electric games that you could get? you had a piece of wire and running from one end of a piece of wood to the other but it was all bent into different shapes and positions. You then had a handle with a metal ring on the end, you then passed the ring over the wire without touching it, if you do it buzzes.
Cleaning sand is a little bit like using one of those games, but obviously a little easier. It just takes practice. Get your siphon going and hover a few inches over any debris, move a little bit closer until the debris starts lifting and then eventually gets dragged into the siphon and then eventually up the hosepipe. Small debris should lift very easily, large debris might mean new putting the siphon a little bit closer to the sand. What type of siphon are you using? What is the diameter? You might find that the smaller the diameter, the stronger the suction. If sand does start going into the siphon, just lift it up straight away and it should fall out.
Just stick with it, you'll soon get the hang of it