I'd tie the alt-wincon into other cards. Do I have a card that mean I get bonuses to creatures from putting large chunks into the graveyard? I'd like to mill for that. Do I have a control card that lets me put the players land on top of the deck, so I can then bin them and set him behind whilst milling?
For instance, card with stats which are some function of cards in graveyards become more dangerous the more milling you do. Cards like: Mortivore and Lhurgoyf
Some creatures gain bonuses or abilities when they attack: Guiltfeeder and Necrotic ooze, or are cheaper to play Avatar of Woe
Some other card will give you extra life, so you can keep milling: Invigorating Falls or Spoils of Evil (whichis more limited, but does give extra mana)
Other times you can pull a card that lets you win earlier than they expect (which will surprise a few people with mammoth decks...), with Mortal Combat
Essentially, tie the player up with other effects and make milling a secondary bonus, that adds to your current plans. Otherwise it's just a one trick pony.