Show Up
It seems obvious, but the way to learn Aikido is: don't stop and don't die. Actually, that's pretty much how you learn anything.
Make it a habit; set a training schedule, say Monday and Thursday at the beginning.
Luckily, Aikido is addictive!
Observe
One of the most important skills you need to develop is the ability to watch the movements of the teacher and repeat them with your partner. You’ll find yourself developing it slowly, though. At first you might only be able to remember which foot moved first. That’s OK. You’ll soon be able to watch a complex series of movements and be able to keep up with the rest of the class.
For more advanced students the problem becomes one of correct observation. Just because you recognize a technique doesn’t mean you can stop paying attention to the demonstration. Teachers are sneaky and might be adding a variation, or even a whole new way of movement. This is especially true in seminars, where a teacher might have a completely different idea of how a technique should be done. One of the hardest skills to acquire is the ability to correctly observe your own movement. If you’ve ever watched yourself on video tape you will begin to see the difference between what you thought you were doing and what you actually were doing.
Do it Wrong
Don't be afraid to do it wrong, a lot. I like the phrase, "Invest in Failure". Screw it up, correct it, repeat.
Mingle
Get a wide range of partners and teachers. Practice with all sizes, genders, ages and species. Aikido with dogs is very frustrating, but instructive.
Learn the Words
It's hard at first, but just remember that the first part of the babble is the name of the attack and the last part is the name of the technique.
Have Fun
Laugh! Enjoy your time practicing and you'll be more likely to keep practicing. We're just primates wearing laundry bags hurling each other on a soft surface.
Help Someone Else Learn
Showing another person newer than you is a great way to clarify techniques in your own mind. The Japanese have even codified this relationship as Sempai and Kohai. Your sempais are anybody who joined before you and your kohais are those who join after you. Oddly, it has nothing to do with rank. There are a whole network of behaviors in how you relate to each other and your mutual obligations.