How to reuse a sewing pattern without damaging it
I regularly reuse my own patterns, as I myself have a few favorites that have been incredibly helpful to me. I use the technique I was taught in a theatrical costuming class:


When cutting the pattern out, cut closely around the outermost size lines (there is no need to be exact at this stage, but try not to leave huge chunks of tissue behind)
Pin or weight the pattern to the fabric as normal
When cutting the piece out, cut the fabric layers only. You'll do this by slipping your shears under the tissue, and cutting while following along with the appropriate sizing line above. If there are areas where there is a single line for all sizes, feel free to cut all layers at that point, but anywhere that's graded should be cut from underneath.


This takes a little getting used to, especially in tighter curves, but with a little practice it becomes second nature. And occasionally (especially in the beginning) you'll nick the pattern tissue, but a little clear tape can easily patch it up, and make it a bit more resilient in that area as well. Similarly, if you pin the pattern (as I do), eventually some of the pinhole areas will start to wear out, but again, they are easily patched with clear tape. I suggest clear over any other options as it makes it much easier to see your scissors as you're cutting underneath the tissue.

When you're done cutting, use a fabric marker or tailor's chalk to transfer your markings from the pattern as normal, patch the paper with tape where necessary, then carefully refold it (along existing lines if possible, or neatly if it's older and the lines are no longer easy to find). You can tuck it back into the paper envelope if the pattern will still fit, or use a gallon size plastic zipper bag to collect the pieces and envelope together for storage.

I have been using this technique, and some of the same pattern pieces, for years. One particularly useful pattern has been reused more times than I can count since 2003, and continues to be both usable and useful.

Sources: Instruction from a costume construction class, nearly two decades of experience.