Are there any tricks or pitfalls in timing gear changes? I have noticed that my bike sometimes changes gears incredibly smoothly—quietly, quickly, very little 'jolt' through my pedals—while other times I get a rough, clunky change. I'm wondering if there's anything I can do to ensure smooth changes, or at least reduce the rough changes.

I don't think there's any adjustments required, as I've had it serviced a few times, and the gears all work (no jumping between them, no really slow changes).

It's generally not a huge issue (I'm commuting not running time-trials) but it can be a bit of a surprise (never good to have your foot slip) or can sometimes cost a lot of momentum when going up-hill.

I'm still getting used to my slightly wider-spaced chainring (new bike is 34/50, old one was 28/38/48), so shifting on the chainring seems to have the potential for the worst changes, but even shifting up/down the 9-speed cassette varies between smooth and harsh.

So I'm wondering if it's a case of timing the gear-change with my cadence? Or do I need to put more/less force on the pedals before/during the change? Or are there other tricks to it? And when I get to the limits of the cassette, is there a trick to smoothly shifting on the chainring &amp; cassette when going up-hill to avoid a sudden ratio jump and loss of momentum with it?
3 basic tips 


Try to be predictive in your shifting. Don't wait until you really need the next lower gear to change gears. Try to do it before your cadence drops to where you're mashing on the pedals
Ease up on the pedals when shifting. If you missed on the first tip, then let up on the mashing very briefly during the downshift. This will aid the chain in transitioning from one cog/chainring to the next. With some practice you can even get the timing right to where the chain will climb (or fall) during the least powerful part of your pedal stroke (6oclock/12oclock-ish) during sprint (or just normal) pedaling
Don't shift the front and the back at the same time. Try to drop the front chainring first and stagger upshifting in the back by a split second. This will help reduce drivetrain issues like dropping your chain and chainsuck.