Id recommend a good few minutes spent by yourself reflecting. Then Im sure that whatever you feel is right will come natural... No matter what kind of mood you want to set, wakes can be pretty important. You want to make sure that whatever you do isnt forced. If comedy isnt what youre feeling, dont try to make people laugh. If you have some half put together slideshow or poster, then dont show it off. Shes your mother... deep down you know what she meant to you, and whatever comes to the surface during the wake will come. Youll just feel what you need to say.
I second what RatWagon has said... funerals are meant to be a celebration of life so whatever you share, is meant to resonate with the people that are there with you. Make them remember your mother for who she really was. Just be yourself and tell it like it is. You cant ever really prepare for something like this, but youll catch on quick and speak from your heart.