I begin to harvest from my sage (salvia officinalis) plants when they are 1.5 - 2 years old. They get bushier and seen to be much more happy than plants left alone. Just take out the terminal leaf pairs down to just above a nice healthy pair of leaf buds/small leaves (opposite leaf structure). Never take more than 1/3 of the living leaves.

My sage grows outside and overwinters in the ground outside. Some folks here haven't had great luck overwintering outdoors but mine seem to do just fine. Put yours in a nice sunny, well-drained site. Drainage is key.

FataMorgana

I have mine in a large pot...it overwinters and comes back nicely the more I cut the bushier it gets.

Sage seems to be a very "forgiving" plant, at least for me. This spring I started an experiment with my 4 sage plants that were about to start their second year. They're in a bed with great drainage and full sun. I left 2 to "do their own thing," and cut the other 2 back essentially down to the ground. All 4 plants are thriving like mad. The ones that were left alone bloomed earlier and got leggy, but I've now discovered that sage stalks will root if they come in contact with the soil. The clipped plants bloomed later, but all 4 are now HUGE. Your climate is cooler, so your mileage may vary, however.