Here are my tips to slipcovers:
1. Go custom with store bought.
Huh??? Let me explain. To buy enough fabric alone to cover a sofa you are looking at atleast $100 or more. Plus you still have all the work of planning and sewing one. I am not nearly that adept of a seemstress, but I do know how to use a needle and thread. I found that sofa cover in a clearance bin for $30 at Bed Bath & Beyond. It was a great khaki color, however I did not see the lovely, huge, purple mystery stain right in the middle until I got it home. That explained the price... But I threw it in the wash with some bleach and it came out a perfect cream color, mystery stain was gone. But it was stilll slouchy and baggy, so I pinned it in a few areas around the arms and sides, stitched it up with some upholstery thread and I had an instant custom slipcover. No sewing machine needed!
2. Pillows are your friends.
Pillow backed sofas are your friends! But if your sofa doesn't have a multiple pillow back and just one or two big ones I would highly recommend finding pillows to match your new slipcover and use those over the cover instead of using the one(s) that came with the sofa. This aleviates the issue of constantly re-tucking in your covers and gives you a much more finished look.
3. Covers that tie at the arms fit best!
Avoid covers like these:
And look for ones like this:
This also helps to tighten the front piece along the bottom, accentuates the arms better, and makes for easy re-application after a washing.
I hope these tips help anyone looking to cover something, especially those who wanted an option to the slouchy store bought covers!