Any suggestions for working on regular light contact?

I am currently training my first horse a 6 year old OTTB of about 1 year, in dressage. We are quite slow at training as with his personality he can be fairly obstinate (read lazy), so to keep him from shutting down, slow and steady training has been successful. The first 6-8 months was working on gaining weight and muscle, and general fun (trails and ground work). After about a year we have gained some decent rhythm, lightening his aid to create forward movement (he used to be very very lazy and ignored aids - we had to go right back to basics), and we started regular contact.

Some days he will just ignore my request for any bit of contact - other days he accepts it willingly and super softly and works from the leg/seat to the hands and works from his back end (with much praise). The other days, he will just pop his head up and go quite flat and not want to relax. Any tips and tricks for encouraging the former? I find transitions from walk to trot, trot to canter he maintains his composure, however downward transitions tend to fall on his front end. God forbid you see our trot through halts.

He is a great horse, and my forever horse, is there anything I can do as a rider to make sure I am not confusing him, or any training excercises that may help? Anything that as a rider I might be doing to confuse him or restrict him? Just looking for more information, since it's always a learning process and always something new to discover or think about :)