For that reason, I usually prefer the stirring method: after spooning a small amount of vermouth over ice and adding the gin, immediately transfer the contents to a prepared and chilled martini glass with two or three good quality flavorful olives - not those tasteless, over-processed Manzanillas.
If you must shake - try this: use a stainless steel shaker prechilled in the freezer. Note: chill the top and bottom separately - it's easier to close an ice-cold shaker than to open one. Add one ice cube only per jigger of gin you plan to decant and drink. Spoon a small - very small - quantity of dry vermouth over the ice and then add the giin. Cap and shake twelve times gently up and down (if you shake it more than that, well, you know what they say...) then strain and enjoy.
You might also want to store your spirits in the freezer. That way the ice and booze are at the same extremely cold temp. As you shake your mixture with ice you get nice little shards that make for a bracing concoction.
Also, after many years of experimentation, I have discovered that my limit is precisely one martini.