In addition to buying new pretreated tick-repellent clothes, you can send in your current outdoor wear and get it treated to repel ticks.
This interactive webpage from the University of Rhode Island answers the three most common questions about tick repellent clothing. Does it work? Is it safe? And where can I get it?
This site provides links to retailers that sell pretreated tick-repellent clothes. More impressive, it includes a feature where for a fee you can send your current clothing to Insect Shield and get it treated as well.