The Erysium (Perennial Wallflower) pictured above is a cutting, one of three, that I took from the parent plant as soon as I bought it. Already I have increased my plants, fourfold, from just one. There is no magic formula for taking cuttings, just snip off a healthy shoot and pop it into potting compost, put a pop bottle (cut in two) over the top, until it makes root, and bingo a plant for free.
So far I have taken Lavender, Argyranthemum. Pelargonium and Dianthus cuttings, as a safeguard, just in case they don't make it through the winter.
This is a good time of year to start increasing your plant stocks and also for a little forward planning; sowing hardy annuals and saving seed from this years plants. I am sure that you, like me, spend far more money than we intend to keep our gardens beautiful, but with a little work, you can keep the costs right down.