The answer to the question in my title is you can't apart from at a General Election. Then the electorate are usually more focused on voting for a party than the actual candidate who, however inadequate, ends up living to fight another day.
Between elections there is nothing you can do even if what he says he will do he contradicts with his actions. The simplest reason as to why what politicians say and do diverge is I believe because in the end they put party loyalty above their own principles and beliefs which are in any event usually pretty flexible! I would also point out that in supporting their party they may well be disregarding the views of the majority in their constituency.
Recently the Tory MP Bob Stewart was interviewed on Talk Radio about the PM's new Tier System and said he had no choice but to support his PM. He however admitted that in his constiuency there were those who supported the measures and those who didn't. What of course he will never know is what the majority felt.
In our second demand 'Real Local Government' we suggest it should be up to each constituency whether they wanted a system of recall and if one now existed in Bob Stewart's constituency then if enough people wanted to they could raise a vote of no confidence in him. Then there would be no question as to whether his support for his party was also accepted by his constituents.
A system of MP recall is 'Real Local Democracy' and, I cannot help feeling like so many Tory MPs, the 'gutless' Bob Stewart might just find that it helped stiffen his backbone to put his constituents first.