how old are the plugs? if old, then try replacing them. also, next time you let it sit for a little while, try letting the fuel pump prime for a few seconds before you start the car (turn the key to ON, let it sit for a few seconds, then turn it to START).
it may also be that your injectors are a little dirty and could do with a proper clean (remove and have professionally clean, not just a bottle of injector cleaner). might be worth changing the fuel filter as well.
final thing is, how old is the fuel in the car? if the car doesn't get driven much, and so the fuel is a few months old, then that won't be helping the problem.