Hi Ben,
I've poured a lot of large molds with bad leaks and I've never had to do more than the following,
1. Use 1 (or more if required) proper clamping mold straps to make sure the mold doesn't move or shift at all when casting it.
2. Use the right consistency of clay to plug up the seams on the outside, fresh soft block clay is about right. This comes with experience but you can feel it going into the seam when done correctly. I rarely do this but if there's a large gap or hole on the inside of the mold that you can reach then push some clay in that as well, obviously use the same clay as you're casting with.
3. Give it 5 or 10 minutes to stiffen a little before casting the mold (depending upon environmental conditions). If you can see cracks it's far too dry.
4. Sometimes the seam will still spring a leak when cast but usually (after finishing the pour to avoid lines) just running your finger along the seam again will reseal it. If not jam some more clay in the leak.
5. If all the above doesn't work the mold is probably too far gone and should be disposed of.
Hope that helps.