I think its a good idea to dismount the amiga from time to time and put all plastic parts in a bucket full of water with some household cleaning agent. You have to remove every single key from the keyboard which will take quite some time. Ive read somewhere that some kind of whitener could be added to the water but i never tried it (dunno if the keys are blank afterwards o_O)
I also dismount the shielding plate and the mainboard and clean it with the vacuum cleaner (be carefull not to scratch it, best thing is to use the brushes that come with the vacuum cleaner).
I do this no more than once a year (maybe if youre smoking you should do it more often).