Now, this is very helpful
sax092 ! Thank you first and foremost for sharing!
So two very immediate things I can tell you that will help.
One: sharpen your knife blade between each cut. Foam is hell on blades after even a single pass.
Two: a great way to clean up your seams is to use a dremel with a sanding cylinder and then use another one but just use the black rubber portion without the sanding drum to burnish your edges smooth. This is a great tool. Heat seal all of your foam prior to putting it together with a heat gun.
Use latex acrylic caulk and a wet finger to fill in your seams and smooth the overall piece.
Use contact cement for gluing together (DAP Weldwood comes in a bottle brush for about 5 bucks at Walmart) brush both sides and wait for it to tack up completely before sticking edges together.
Use Hot Glue to reinforce seams from the inside.
V Notch bends and bevels on the backside of your foam to help shapes stay in place. Fill these gaps with hot glue.
These few things are a huge help. Watch Bill Doran over at Punished Props videos and Evil Ted on Youtube. Both do amazing Foam Work.
These few things will move you light-years ahead! You will get cleaner pieces in foam.