Limeycricketlover,
in order to retain water, you need sodium. So watch your sodium intake carefully to keep the water rentention as low as possible!
I didn't gain weight from the Prednisone (ok ... a lil bit over Thanksgiving, but that has nothing to do with the Pred ), but I have cravings ... which I don't give in. I learnt to ignore the cravings last year when I quit smoking and feared to gain tons of weight! But if I do crave, I crave hearty and salty foods like french fries, chicken sandwiches, lots of ketchup on top of almost everything, gravy, everything with melted cheese and carbs! Crazy for noodles and bread.
You mentioned increased pulse/heart beat. I have that too (sometimes palpitations and it feels as if my heart is skipping a beat), but I am not sure if that's from the Pred or from anxiety and nervousness. I got worse since I started the 6-MP, but it could also be that I am nervous about taking 6-MP and anticipating bad side effects (I know, I shouldn't borrow sorrow, I am a worry wart, I try to change)!