I usually try to get about 3 or 4 stems at once and put them inbetween my middle and index finger with the roots on the palm side. Then I stick my hand into the sand and just dig in spread my fingers so the stems drop and the sand from the other side slides in to fill the hole. Then I'll move a bit more sand to the area I just planted to help hold them down.
Stubborn plants might require a weight or well placed rocks/slate.
I use some big tweezer like things to do my planting. It works much better for smaller size substrate, like sand. You need a fair amount of sand to be able to hold the roots down....atleast 1.5" or more...and shove the plants as far as you can into the sand. With larger rooted plants (like swords, crypts) you will want more sand to hold them down.
I haven't planted a sandy tank yet.. . but I have found with eco-complete that if I plant the plants without water, and then add water, they stay put a lot better. Planting in a full tank is a real pain.
i have about 3 amazon sword plantlets in sand and they seem to be doing better than my plants in the pea sized gravel... i just leave them be and they do ok
well, sand is the best substant to keep plants down, you can try to tie it down with fishing wire, or weight it down, but for me, just pust them in the sand, they'll be just fine