I often will try and put myself in the others guys shoes and think what he would probably accept that i would also be willing to part with. Then I whittle it down a little so I have room to move before sending an offer, it shows I've thought about it and I'm not just trying to rip him off.
On the other hand I often send out feelers to see if players I want are on the market, the odd email asking if a GM is even willing to talk about trading a given player can prove fruitful. Not necessarily a "who do you want for Stamkos?", but rather "is Stamkos on the market at all?" I find you will often get a more positive response.
ASKL - 10 Team dynasty,G/A=1,PIM=.1,+/- =.2 SOG=.05 (W=2,SO=3) Keep 12F 6D 2G 8BN + farm
Farm- Namestnikov,Khoklachev,Rundblad,Despres,Jensen, L.Adam,Barberio,Galiev,Telegin,Hertl,Aberg