As last season's winner I feel I should chuck in my opinion!
Yes lets do only Premiership games.
Lets do ALL of them though.
Lets have monthly comps, from the 1st to the 31st.
Lets charge £5 per month, and pay out 50% to the winner, 25% to 2nd and 15% to the site, that way you are not asking the winners to "donate if you want to".
They can if they want of course.
That leaves 10% to pay out at the end of the season to the overall winner.
Instead of adding all points up at the end, award 5 ,4, 3, 2 and 1 point to the top 5 each month.
This way, if you have a bad month, or you forget a couple of weeks, then you wont lag too far behind and be tempted to drop out!