A suit that is too big is COLD. The suit should fit very snug and should when dry feel a bit too snug. I have made the mistake an my last two suits getting one size up to be comfortable out of the water and found out fast that once wet they got even looser and I had water pooling around my knees, ankles etc.
I find for short back zips and front zip suits thay if you pull the waist up threw the opening at the top is make the opening wider and you can get the suit past you hips easier. After that it just a matter of getting you arm in it and over you shoulders.
On front zips I usually start on the side the the ziper sit when the suit is open (for expample In 5/4/3 flow is zips from right to left.) I get my right are in up the the shoulder and work the left arm in after slowly .
For short rear zips I put both arms in and work it evenly up over my shoulders.
Getting them of is best to peal it off slowly working it down . Dont try to get alot off to fast or at once all you do is strech or worse case tear the suit