Kayak dollies are pretty simple transport carriers that come in a variety of shapes and sizes.
Tip #1: Budget. $15 - $200 price range depending on if you need a fixed dolly or compact / collapsible one. A fixed dolly will be at the lower price range and is a good choice if you are looking for a basic transport carrier and not too concerned on storage size. A compact / collapsible dolly should be considered if you have limited storage space and / or you will be taking your dolly with you when you launch your kayak. Many compact / collapsible dollies are designed to fit into the kayak hatch storage areas - a great feature for touring kayaks. Look to pay more bucks as your dolly size decreases.
Tip #2: Wheel type and size. Dollies can be made with bicycle or bungee inflatable wheels. Bicycle wheeled dollies are good for a variety of terrain - rocky, uneven, paved, dirt, and semi-soft. Bungee (ballon) wheeled dollies are best for beach environments - especially soft sand. Decide what will be your typical terrain when launching your kayak and buy accordingly.Tip #3: Where it attaches. Dollies are designed to attach either at the stern or mid-section of the kayak. The stern attached ones usually are less expensive, but you have most of the kayak weigh when pulling it. Mid-section attached dollies distribute the kayak weight more evenly making it easier to pull.
Tip #4: Talk to a good sales person at a kayak / canoe dealership or an outdoor provision store like REI. Many of these persons have experience with kayaking and can be very helpful in sizing up the right kayak dolly for you.
Check out this video to get an idea of how kayak dollies work. Not endorsing this company, but does give a good demonstration of a collapsible dolly.
Do you use a kayak dolly? How does it work for you?