Bundt cakes are made extra special by the shape of the pan they are baked in. While they all have the standard open tube down the middle, the design and shape of the sides can be any number of things. You want to take every precaution to ensure that your cake comes out of the mold in one piece. Here's the best way to prevent sticking.
Butter and Flour to the Rescue
Nonstick pans are important, but don't trust only a pan. The best prevention is actually butter and flour, which you probably have on hand from deciding to bake the cake in the first place.
How to Do It
Before you start making the batter, coat the pan (and I mean every nook and cranny) in a thin layer of butter. Sprinkle the butter with flour, tipping and shaking the pan as necessary, to make sure all of the butter is coated in a thin layer of flour. Discard any excess flour and then make the batter. As the cake bakes, the butter-flour coating will create stable sides for the batter to cling to as it rises.
You might be saying to yourself that your nonstick spray will do the same thing without as much effort. While nonstick spray helps with casseroles and other baked goods, it hinders the baking process here. The slippery sides will make it hard for the cake to grip, so it won't rise quite as high as it would have with a butter-flour coating.