(I'm adding this as an answer, because it addresses some points in the original question as well as supplementing the tips given by other answerers, but although it's kind of supplementary, it is too long to be a comment. If that's not correct SE etiquette, please let me know, but I thought the guidance was important enough to be added here.)
Avoidance through terrain choice
While a lot of good advice has been given here about dressing, self-inspections, and chemical repellents, I'll skip directly to your question about avoidance: while found everywhere, ticks prefer warm, damp climates and tend to be most populous on islands and coastal areas. Going inland and to higher ground (where there is less vegetation for them to hide in, and also fewer mammals to attract them) would help here. The CDC has an interesting set of species distribution maps for ticks in the USA. In short, weather and location have a great effect on your likelihood of encountering ticks.
Avoidance with darker-coloured clothing
Some studies have found that ticks are more likely to be attracted to you if you wear lighter-coloured clothing. Darker colours, on the other hand, seem to be less favourable.
One item which I would always carry in an outdoors first aid kit is the tick removal tool. They greatly reduce the risk of squeezing the tick when attempting removal, as can happen with fingernails or tweezers, which can force it to vomit inside the bite. They do this by being specially-designed to grasp the tick firmly, while avoiding lateral pressure on the tick's body. There are two main styles of removal tool:
- the fork type, where you slide a narrowing V just behind the tick's head
- the pincer type, which has a spring-driven pair of pincers, and continues to hold the tick after removal
With or without a specialised removal tool, tick removal technique is still a much debated issue: should you pull straight, or twist as you remove it? The current position of the CDC is to pull straight. This is because twisting can cause the head to break off (the exception here is removal tools which are specially designed to use a twisting motion), and thus presents a lower risk. The general guidelines are:
- the tick's body must not be compressed, as this can force out saliva and gut contents which may contain disease-causing organisms
- the tick should not be irritated or injured, as this may result in it regurgitating (vomiting) saliva and gut contents along with any disease-causing organisms
- the mouth parts of the tick should be cleanly removed along with the rest of its body
Keep the specimen in case of infection
If you have any concerns about the nature of your tick bite (or especially if you see any redness around the bite area - concentric red rings are a sign of Lyme's Disease), seal the tick in a Zip-Loc bag and freeze it. This is because in addition to Lyme's Disease, ticks can also carry and transmit dozens of other nasty things (also listed at CDC), including bartonellosis, ehrlichiosis, encephalitis, ricketts, and more.
Bringing the source tick to your hospital if you find an infection can help the medical staff to quickly identify what the creature might have been carrying.