8 ohm polarity question...

I often have to wire speakers that are two 8 ohm on one channel and two 4 ohm on another... It always seems like when I put the two 4 ohm speaker together in series that the polarity gets flipped. I end up putting the negative to the positive of the amp because it sounds better... Is there some sort of rule to use when wiring this kind of setup so that it comes out the same polarity as the other speakers.

Also sometimes i have to wire tweeter to these same channels... is there a specific leads to use to get the positive and negative correct?

thanks,

-J