One way to think about training is to "train for the sake of training"...
A pretty good book on this subject is George Leonard sensei's "Mastery."
As usual, I really have to agree with Jun in this respect. I've been doing aikido for some 4.5 months now, and I have never once found myself frustrated to the point of quitting. I think the secret is to live in hte moment,and just, as jun said, train for the sake of training. Train to get better, but not to become shodan in a week. You will become a yudansha when you become a yudansha. And that little black stripe across your middle doesn't mean anything if you're not a dedicated, good aikidoka that can asist beginners in becoming good aikidoka. So don't worry that you've been doing it for X amount of time and you aren't X rank yet. Just enjoy it for the beautiful and enriching art that it is, for the kindness and compassion of your sempei and kohai, and for the fact that you're learning something. I've never attended a class yet where I didn't learn anything, or didn't teach someone something(equally important in my book)- and that is how I avoid being frustrated.
Hope this helps,