Hmmmmm....not exhaustive by any means but heres some things I've found in the past:
1) Water in the fuel tank
2) Water in the Carb bowls
3) Crud/deposits in the fuel lines (and the extra fuel filters on the Carb bodies themselves)
4) Carbs out of balance
5) Worn carb internals - Emulsion tubes and needles appear to be favourite.
Plugs - well I've tried NGK Platinum plugs and found a 'slight' improvement but they were almost £10 each (there are some on Ebay for about £8) or try:
www.sparkplugs.co.uk Leads: I imported some from the USA (again via the dreaded Ebay) and cant say I noticed any improvement.
Good luck - Frank