the mailing preference scheme helps but if you then fill your name and address in on some direct mail campaign or put it on a newspaper coupon or summit, you can easily undo the work you did contacting mps;

if you want to avoid junk mail never put your name and address down for a brochure or complete a guarantee card for your toaster or give it to the electrical store for 'the extended warranty' or anything else like this.

they will tell you "it's for our records" or "we need it in case you make a warranty claim", (had this one for a £35 bycicle in halfords recently).

"we promise we wont mail you", "dont worry sir XYZ does not send out junk mail (but we will sell your name and address and not tell you we have)"

they all do it and many simply igfnore the data protection act.