Here's another technique -
1) Put the text layer above the sand layer (in this case you'd probably want to draw your name so it has that hand-drawn look).
2) Add an inner shadow effect to the text and turn the fill of the text layer to zero.
3) Look at the channels of the sand layer; find the one that has the most detail (nothing too blown out or too dark). I tried a sample sand image, and found that the green channel had good detail. Copy this channel and paste it into a new document.
4) Run levels on it to increase contrast, but not too much, you don't want wash out any detail.
5) Sharpen the image; I think over-sharpening is good in this case; you almost want the grains of sand to pop out.
6) Run a tiny bit of blur; the sharpening brought out little details, but now you want to blur a bit so that the transition for the next step is smooth.
7) Save the file (as a PSD).
8) Going back to the original image, select the text layer. Use the Distort, Displace filter and choose the file you saved. Play with the settings.
You should now see your text on the sand, and it should roughly conform to the shapes of the sand. Here's an example.
Writing in the sand <
http://www.mustangmods.com/data/8259/sand.jpg>