My westie is nearly 5 months old now. He's losing his puppy teeth and getting broader in the beam!
We notice that the command for sit seems to have fallen out of his head and he's taken to climbing up the back of the sofa and jumping onto the seat. He still whines if he's left in another room by himself (say if we're trying to have dinner in peace without constantly telling him to get down).
Is he becoming a teenager - how do we re-train sit? He'll do it for a treat but that's all, and won't 'stay' for longer than a few seconds.
At the moment we seem to be suceeding with ignoring the hysterical jumping up when someone comes home - that's improving but it's taken ages. He gets totally over-excited when people visit and can't be let in the same room if they want to sit for a chat, so we put him in the other room and of course he whines like crazy. I've had to start ignoring him some evenings for short periods cos he wants to play all the time and will sit in front of me whining. Hopefully this will take effect as I notice he now sometimes plays on his own for a little bit with his (many!) toys.
We don't want to install a baby-gate as we may be moving soon tho I know that might help with the whining.
Thanks - I'll stop whining myself now :)

Get hold of "The Perfect Puppy" by Gwen Bailey to start, you'll find lots of good advice there :)

She's done another one called "Dogs Behaving Badly" which is also quite useful and has good info.

If he will only sit for a treat, you may ave kept the treat in view for too long ... try asking for sit, hold the treat and reward...then ask for another one, but withi the treat on say the worktop so that he can see it but it's not in your hand. He may sit, if so give him the treat; if he does not, just say "oh dear" and walk away, try again 5 minutes later. Eventually he should get the idea but it may take a few tries.

It's good to reward him but in time, ask for several repetitions before he gets rewarded, and sometimes reward with praise, a toy or a game as well as food so he neve quite knows what his reward will be, this will keep him on his toes more ;)

Hth