the more you play the more your range improves...along with tone and shiz. one thing though is consistency. i know if i miss practicing for one day (all my friends can attest to this) i sound like complete crap 2 days after my missed day of practice. Bigger mouthpieces are rly good. schilke 51/52d's. bach 5g. those are all good for baritones. lip slurs rly help ur range. long tones help alot too. they strengthen the corners of your mouth alot provided you have the right embochure...if u dnt then you're just wasting your time.
air is a good thing to have too. use alot..its free. breathing excersice are nice to have. as far as excersice books go, get an arban's or be a bum and "borrow" your friends book and make a copy of a page and play it till you master it.
have fun