Walls and Ceilings - Keeping mud lines straight

I am attempting to mud an inside corner that is about 130 degrees and I'm having problems keeping the mud line straight. Is there a tool or technique for doing this? Thanks.

You need to use either straight flex tape or a product called no coat. If you have just the one corner then probably best to use straight flex. It is very important to bed the straight flex in quick set mud. You load the corner up with the mud, cut your straight flex to length, pre bend the tape, then set it. Once you have it straight then wipe it down putting most of the pressure on the outside of the knife. Then let it set up. After that you should have a nice defined straight line to mud to.

I agree with coops28 on the straight flex. I am in the middle of installing 4 trey ceilings in a basement for a customer, and the straight flex is allowing me to strike straight lines all around the room with the 45 degree corners.