How can we prevent unrelated earmarks or pork barrel spending in bills?

I understand that some of the spending measures inserted into bills are good or beneficial, but I would like to see more transparency. I think of this because of the Iraq funding bill presented to the President by the Democrats. Would the Democrats not look better if they didn't fill the bill with all sorts of unrelated spending? Aren't they just giving the President rationale for vetoing? Why aren't they lauding the other points of the bill, or do they feel safer keeping it silent because most of these benefit constituents (buying votes)? The American people just seem to only get a slice of the picture, most of the time. This goes on with both parties, no one is exempt.