Well, there's a few tricks I know of that can speed things up.
1: Set your router's channel to 1 or 11. Rumor has it that this speeds up the connection, so things go faster and there should be less lag.
2: Don't use any computers when on wi-fi, and close any and all internet-using programs. This lightens the load of all the data being sent, allowing the router to communicate with the Wii faster.
3: If you don't have a 3 bar signal, move the router/wii closer to eachother. Simple enough, yeah?
4: Try to make sure you're playing with people in atleast the same country or at worst, same continent as you. Long distances, or especially overseas wi-fi communication can be rough for the Wii.
That's all I can think of for now... >_>;