Tyre pressures, theoretically before every ride, but TBH I don't know anyone that actually check's them - worth doing at least once a week though as off pressures can radically affect the bike's handling.
I have some "traffic light" valve caps so at least I can see easily and quickly if there is problem if they aren't being checked every ride.
You buy them for the correct pressure range and they should stay green, if it does amber or red then you know the pressure has gone down.
As none of us are perfect at doing this every ride then I think this is a good alternative for a few quid.
This is an example
http://www.ebay.co.uk/itm/MOTORCYCLE...item335f34351e
I haven't searched on price and that's not a recommendation for that particular seller, just an example of what I mean.