The first step is to determine how far you're moving and what size of truck you need. Some rental agencies will charge a flat daily rate, including a determined amount of miles, while others will charge an inclusive rate. Know your mileage and size of truck you'll need before you start your research so you can shop around and compare prices.
Remember, your truck should be large enough to accommodate 10-15% more than what you own. This is just to ensure you're going to have enough room. Too much room is better than too little.
It's also a good idea to determine how long you'll need the truck (how many days), when you want to pick it up and whether it's a one-way or two-way move. Many companies charge a premium price for one-way moves; this is to cover the cost or retrieving the truck.