I'd definitely only go up one tooth at a time. It makes a noticeable difference in raising the overall ratio and making all the gears taller. I've been using 18/42 for 5 of the 6 years I've had my bike. I sometimes think about going back to the 17 as I really liked the more aggressive starts - probably why Ian likes the 16. Also, if you're not used to a bike that likes higher rpms, the tendency is to shift too soon.
A 19 tooth front sprocket would be way to high for me, but some here like it - especially for highway riding. Think of it like gaining a 6th gear, but always starting off in second. The point is that a little goes a long way. Try going up one tooth, but be sure you're not shying away from revs.
After 22,000 miles on the 18, I may just go back to the 17 when I feel like messing with changing it. I'd run a couple of hundred rpms higher on the highway but have more fun taking off - especially seeing I ride 2-up a lot.
