Contacting your MP is always worthwhile, although some are more or less interested in anything consituents have to say. Most will reply to a coherent well-argued letter, although responses can range from surprisingly attentive through to... well, have they actually read what you wrote?

If you can, don't just email. It's much easier for you, but also much easier for them to click past. A proper letter on paper sent through the post, or even a fax will command a great deal more attention. Recorded delivery is best of all - they might think it's from the Fees Office and open it promptly ;-)

   xxxxxxxxxx xxxxxxxxx MP
   House of Commons
   London
   SW1A 0AA

is all you need to get a letter to the MP's parliamentary office.

However it is much better to send to MP's constituency address if possible. It makes it clear you are a voter on their patch and it is personally relevant for them to pay attention, that you are not just some nutter with a gripe or tedious PR twerp who's spamming all 700 of them.

Finding your MP's name and contact details is generally easy, although a few seem to be in permanent hiding.

The official Parliament site will find MP's names and constituencies http://www.parliament.uk/mpslordsandoffices/mps_and_lords/alcm.cfm
This lists email addresses and websites where available (most have both). Their websites usually give constituency office contact details.

Alternatively use any of these, which also give opposition candidates details

http://www.theyworkforyou.com/mp/
http://news.bbc.co.uk/democracylive/hi
http://www.guardian.co.uk/politics/page/2007/dec/18/1

And this one allows you to email MP's, Lords, MEP's & councillors straight off the site.
http://www.writetothem.com/
You can't send attachments though, and MP's sometimes ignore, or perhaps never receive emails sent via third party sites such as this and theyworkforyou.com

CC'ing to opposition candidates is a good idea too. If enough think your issue presents a good opportunity for beating up HMG it might even get taken up at a national party level.

http://www.theyworkforyou.com/mp/ is a particularly useful site if you also want to know how often your MP shows up in Parliament, how they have voted, and what they have said in Parliament. You can also set up keyword searches so that any mention of a particular topic in the House of Commons will be emailed to you along with links to the debate.