There are a couple things to do once your joint starts to canoe (burns unevenly), but let's try to prevent that.
The first thing to help prevent canoeing would be to use a grinder to grind down your herbs. Using a grinder will allow for your flower to be consistent while burning. The next thing that will help is not rolling too tight. Having a good consistent air flow through your joint is key. Last but not least, start with a full, even light of the joint.
Sometimes things happen and our joint gets away from us and take a wrong turn and start run/canoe. When this happens, simply tap the fire (cherry) of the joint in the ashtray till the fire on the joint is even. Then, slowly puff the joint to resume an even flow. You may need to re-light.