This isn't an original idea, I stole it from a product I saw at REI that sells for $16 for 4. I made 4 for less than $3.
DIY BEACH "Stakes"
These work great to secure a tarp (or tent ) in sand when regular stakes just won't cut the mustard. Field tested this weekend (on a ludicrously big tent in constant steady 14-20 mph winds). Didn't budge at all.
Cut 4 12x12 inch squares from and old sheet, pillow cases, whatever. I went to a thrift store and bought a sheet for 50 cents. Tie a simple knot in each corner. Cut 16 3 foot lengths of cord using a relatively thin, yet strong cord. I used some Coghlan's camo cord from WalMart which cost me all of $1.88. Tie loops in the end of each length of cord, the resulting loops should be roughly 3/4 inch. Tie a length of cord behind a knot on the 12x12 square using a lark's head, pull it tight so the knot forming the loop passes through the loop assuring the hitch doesn't come undone. Repeat on each corner. Gather all the cords and tie a loop in the end.
I know the written instructions are a bit convoluted. If you can't figure it out from the picture, and you still want them, get them from REI.
To use, dig a hole, fill with sand, drop in hole, cover with sand, pack down tight. Ta-da.
Also, bring a plastic bag to put them in as they will be filthy with sand after you pull them out.