What can I do/take to lessen post-workout soreness?

After gaining a significant amount of weight over the last 18 months I joined a fitness center yesterday and am paying (a lot!) for a personal trainer. Having never been into resistance training all that much I was surprised at how sore I am after only a 30 minute workout. I have another session tomorrow and I’m fearing it already because I’m so sore. I took two asprin but it didn’t seem to help. The trainer told me to drink a lot of water and stretch. Anyone else have any suggestions for quelling the soreness?