Make sure your ingredients are at room temperature before you blend. Using a cold egg with room-temperature oil could cause problems with emulsification.
Add a tiny bit of fresh whey to the jar before you blend and your mayo will last over a month in the fridge since whey can act as a natural preservative.
For mayo lovers - triple the batch and make it in a quart jar if you go through lots of mayo or will be making salad dressings, pasta salads, potato salad, or deviled eggs with your mayo.
Get creative with some add-ins - try a teaspoon of chipotle powder and some red pepper flakes for a spicy mayo, or garlic and herbs for a delicious garlic-herb mayo.