Although im new to Aikido, i have noticed how weight lifting has hindered my performance during a learning session. I know many aikidoka have a problem with keeping their shoulders lowered and relaxed, but weight lifting has seemed to make it worse for me. If you are organized properly, I believe both weight lifting and Aikido can be done together. I would recommend not weight training on the days that you are going to Aikido, and make sure you stretch all muscle groupings until they feel loose before and after you weight train. The problem with weight training is that it leaves you sore and stiff for the follow few days, so a good stretching routine can greatly improve your ability to perform well in Aikido.
Also, follow Lynn Seiser's advice. => Great post!