How can I make my slipcovers fit better?

You can adhere Velcro to the underside of the slipcover and the couch. Place them in a few strategic spots you can get sticky back velcro from most haberdashery stores.

I tuck everything making sure the fabric stays even.

Hi 516: Well, if you sew, you could sew on some of that non-slip shelving material along the edges. I have used it under carpets, cat dishes, tools and lots of other things, except in my cabinets:) The dishes don't slip out easily, so it works on other things. Just cut the shelving material into about 2 or 3 inch strips and sew it on both long edges. If you don't sew, you may be able to just tack it on by hand, in several places. Good luck

A couple of good tips here!

https://homeguides.sfgate.com/tuck-slipcover-couch-55912.html

Hi! I immediately thought of this tip! Good luck!

https://www.hunker.com/12446321/how-to-use-pool-noodles-to-secure-sofa-slip-covers

Put elastic all around the edges to stretch fit then ties on the bottom so you could tie the frount and back together so it doesn’t move

Put a slip cover on your sofa. Then push a broom handle down into the space at the bottom of the back. Put the covers on the seat cushions and put them in place. The broom handle will help keep the covers in place and nobody will even know it's there.

Use some upholstery twist pins to hold it in place https://amzn.to/2Fz9r4A