That is one of my favorite things to do. Stock doubles, sell half, basically free shares depending on if in a taxable or retirement account. House money!
I think I posted something about a buddy that would buy 100 shares, if it went up 6-10%, he'd sell enough to cover his initial investment and wait for it to cycle down to find a new entry point. His mantra was share accumulation was the key to success. He did this with a lot of energy companies, exxon, chevron, hess, slob (SLB) since he was familiar with the oil patch. Toss in dividend reinvestment for each and nice returns over the years. The kicker of doing this is that his TRUE cost of the shares was his tax liability on the gain. If the shares were $100 and he was in the 20% bracket, his taxes were $20/share. Once he got 100 shares sell a couple series of covered calls or puts and true free shares.