I've yet to master the revitalization of flush ball seals, always had to replace them especially when the kids didn't understand the concept of keeping water in the bowl.
Last time I looked the seal was around $10. Not hard to replace, just look at your toilet's owner's manual for directions. If its a decent quality dometic thats in good shape otherwise its usually worth the time to fix. If its an old plastic creaking special I always upgrade to a 310 or 320 model. Walmart sells the 310 for $199.99.
I second the motion for using silicone grease. Just make sure the seal and the knife blade are dry before applying a liberal amount of silicone grease to both. I have used this method for years and it saves a lot of work and expense.