[Morecambe Ex Pat]

If they ring you at home it is a free for all for you to take the wotsit.

You can either get annoyed or make it into a game.
If you are busy and it is nobody you know - just put the phone down

If you have the time - let's play

Surveys
If they are doing a survey, ask how much you will receive from them for doing the survey. If nothing - rant and rave about why should you work for nothing when they are being paid for talking to you.

If they are selling - tell them you already have one, it is green and has been filtering your fridge magnets for over a year and costs less than 3p per day to run and with the savings in green shield stamps it turns a tidy profit. Perhaps they would like to quit their job and go working for you.
Then proceed to sell them your fridge, car, bed, wife, husband, kids or anything else that springs to mind.
Most of these people work off a script and the measure of your success is getting them to put the phone down - try it - they will not ring back.