I have a shower stall in the basement and a tub/shower on the first floor both of which are rarely used. In order to keep the sewer gas from coming up I normaly run some water down the drains but ususlly don't remember till there is an odor. Is there a better way like some sort of plug or something other than water to pour in drains that won't dry up as fast to solve this problem?

A few ounces of mineral oil down the drain would stop the water evaporation. Don't use motor oil - it is polluting and should not go down the drain.