What I do if they are blocking the bag, is go in feet first and get my heel on their foot or ankle, and my toe/rest of foot on the bag. Very effective in making sure the bag remains open for the duration of the game. A little bit dangerous with spikes, but in my summer league we have to use plastic which sucks, part of the reason I tore my acl, but I guess because no one can get spiked by things like what I do. Another thing that my coach taught me, is that if they are blocking the bag, when you dive in, use your forearm and just give a nice hard jab into the side of their leg. If they don't fall and miss the ball, they will be hurting, and will not be in the way anymore, just make sure it is not blatant that you are trying to hit him, just make it look like your going straight for the bag. Sometimes the game of baseball can get dirty.