I always have a kind of "warm-up" before each training session. Before teaching a new trick, I go through all(or at least some) of my dogs' already known tricks. This gets their focus on me, and improves the tricks they already know. Sometimes during play I'll randomly ask for a behavior, just to see how they react. I think it really helps when I actually begin a training session and ask for that behavior again.
Pay more attention to your body language. For instance, if you are subconsciously swinging your hips a certain way when you ask for the backwards weave, he's probably noticed(even if you haven't) that your walk is extremely similar. When he tries to go into the backwards weave during the heel, simply assume that he has temporarily forgotten how to heel. Go back to using the same method you used when you first taught him to heel. Perhaps give a verbal, "Noooo" or "Ah," not firm as if to correct him, but simply something to let him know that's not what you're asking for. Get in a position where he can't backwards weave, like with your legs together. Then ask for the heel again, and incorporate your original training method. You're not exactly reteaching the behavior, you're just giving him a refresher course to help him understand the difference between the two behaviors.
Here's an example: You go for a walk, and your dog is constantly trying to weave through your legs. The next time you work on the weave, pay attention to your stride length. If you've been letting him rely solely on your body language, then he's just doing what you taught him. Make your weave stride longer than your normal stride so there is a clear difference. Incorporate the verbal cue and hand signal more, so that he doesn't confuse your mistakes in using your body language. Being completely in-tune with your body language takes lots of practice and time, so just be patient and keep at it. Hope this helps! Good luck! :dogsmile:
Edit
Another thing I thought of...
With some behaviors, it's beneficial to "wean" your dog off of your body language cues, so that it takes less of a movement to get them to perform the behavior. For instance, I taught Mud to back up by holding a treat over her head, moving it back, and walking towards her. As she improved, I would move more slowly towards her, or take less steps. One she was responding well to this, I would just lean forward a bit. For her, I added the hand signal later. Now, I've completely elminated the body language. I don't lean at all, unless she gets distracted or something, but I rely mostly on the hand signal and verbal cue.
By the way, Alfie's gorgeous! Just watched your vid on your profile. ^^ He's great. He looks very snuggly, haha!!! :doglaugh: At the moment my Border Collie is laying beside me trying to lay her head on my laptop, so it's been a little challenging to type this, lol. :dogrolleyes: