According to most research I've seen, DEET is not considered a very good tick repellent. I'm also surprised that no one has mentioned the use of 'permethrin' based products for clothing treatment, which seem to be the most effective tick repellent currently known.
Note: Permethrin based products are for clothing treatment only and should never be applied to the skin! Although permethrin is used in many insect sprays and sounds like a major toxic chemical, it's made from chrysanthemums.
By treating the outside of the clothing you remove the need to apply anything to the skin. Not only does this eliminate problems associated absorption of DEET through the skin (most DEET products contain alcohol which contributes to this), but it also negates the need for re-applications due to sweating. An added benefit is that treated clothing can be washed 2-3 times, and the treatment will still remain considerably more effective than the use of a DEET based product (over about a 2-3 week period).
And of course follow all the basic rules. Wear two layers of wicking socks where your pant legs are placed between the first and second layers, tuck in your shirt, wear a hat, and make sure all insect entry areas are treated with the proper repellent (shoes, outer sock layer, lower pant legs, belt-line, and hat [see note]).
Here's an example of how safe it's considered to be (the first product).
http://www.bugpage.com/icp.html
Here's an MSDS on a permethrin based product. Keep in mind that EPA MSDS data sheets always describe 'worse case' situations.
http://www.scs-mall.com/images/sp7msds.pdf
Here are a few links for more information:
http://www.lymealliance.org/tick/humans.php
http://www.lymealliance.org/tick/repellent.php
http://jama.ama-assn.org/issues/v285n1/ffull/jlt0103-3.html
http://www.lymealliance.org/tick/removing.php
http://www.cdc.gov/nasd/docs/d000801-d000900/d000846/d000846.html
http://www.travmed.com/trip_prep/insect_permethrin.htm
Note: There is mention of treating hats on the first link's site, which I do also. However, it's important to not apply any permethrin treatment that may come in contact with your skin such as any area on the under side of the hat closer than 1"-1.5" from where your skin may make contact. What I do is spray the entire top of the hat, turn it over, and then place a mask I made out of cardboard that shields all but the last inch or so of the brim from the spray (depending on hat style).
Where to buy. It should be available at most sports/outdoor shops, but here are a few links:
http://www.bugspray.net/catalog/products/page15.html
http://www.forestry-suppliers.com/product_pages/View_Catalog_Page.asp?ID=6601
http://www.forestry-suppliers.com/product_pages/View_Catalog_Page.asp?ID=5997
http://www.rue.com/repellents.html
http://www.wholesalehunter.com/product/duranon/Duranondes/DuranonTick.htm
You can also buy this stuff in concentrate which is usually 13.3%. Some sites that sell such concentrates state you can use it full strength on clothing, and that it will last for several months, but in my opinion you should not mix it in stronger formulations than the recommended (.5%). In other words, 1 oz of concentrate to ~24oz of water, or 5.5 oz per gallon of water. Certainly no more than 1% under any circumstances. Obviously a more liberal application will increase the actual active ingredient percentage on the clothing so this is probably the best way to achieve a higher concentration if desired. Normally a .5% solution lightly applied is quite effective, so a heavy application is not normally required.
http://www.travmed.com/scripts/catalog.epl?product_id=173&category_id=44&moveit=4
Anyway, I hope you find this information useful...