I usually use hog rings for summer sausage but when I do have to tie with twine I use a double overhand knot. It can be a bit tricky if tieing by yourself though, could try a slipknot then you could tighten with one hand while holding the twisted casing with the other hand.
Places like cabelas have the pliers and hog rings. The last batch we did I used plastic zip ties. Which you can buy from any hardware store. Just as a safety precaution (wasn't sure how the ties would hold up to heat) I also tied with butchers twine. The zip ties made it easy to close tight and they held up to the heat just fine.
went to lowes today to get some hog ring pliers, those people there did not know what hog rings pliers were, finally found them in the fencing dept, those pliers were BIG and had 1 9/16 inch rings (only size), well i guess i will be on amazon, to try to find them. or lem, but there shipping prices are way to high.