Here is a quick guide that will hugely reduce the chances that your domain will be snapped up by someone else:

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• Make sure you are listed as the Administrative and Technical contact: If you are listed as both then any changes to your domain will have to come through you. Plus, you can make whatever changes you like without being vetoed, stalled or held to ransom by someone that doesn't want to lose your custom.
• Make sure those contacts are correct: Check in particular that the right email address, right physical address and correct phone number are on your contact list. You can check this easily on any whois service. The email is particularly important since this will the first port of call if any changes are about to be made to your domain. Check them regularly - make it a monthly habit.
• Go to a reputable registrar: Yes, others may be cheaper but you may also find they go bust, that they don't answer calls, that they have poor security, that they won't change information readily, that they charge you unreasonable charges - the list goes on. Do some research. A few extra quid is worth it.
• Try to choose a registrar with an online control panel: Calling up registrars and dictating your needs (and then faxing them on headed paper) is time-consuming, frustrating and slow. If you choose a registrar that lets you do everything online, you aren't constrained by employees or opening times or constant procedural delays. Also, whatever you do, choose a difficult password - a combination of letters and numbers. Otherwise someone could get in as you and sign all your domains away legitimately.
• Check the registrar charges: Ask them all about their charges. Ask what their services are, what they mean and how much they cost. Ask if they charge for making changes. Some will offer enticing Gold, Platinum, Plutonium accounts. Do you really want this? Chances are it will include all sorts of features you will never use, so why pay for them? If you don't understand what something is, ask for it to be explained and don't stop asking until you grasp it.
• Get a registrar that locks a domain: This is a process by which changes cannot be made on a domain without a significant level of checking. It was put out in response to the worrying increase in domain name hijackings. But of course domain locking has now become an additional feature. Makes sure your registrar can lock a domain and make sure they do. If they charge for this, tell them where to stick it - this service nearly always costs the registrar nothing.
• Check the registrar's security: You wouldn't expect your mortgage company to sign over the deeds to your house to a man that walks into its building with a credit card saying they were you - and so you shouldn't expect registrars too either. Ask them about the security they offer. Ideally, you would want email, written and aural confirmation before major changes were made to your domain. Or, of course, digital certificates. Or passwords if you use an online control panel. Make sure you have the level of security you want.
• Do NOT let your domain expire: Whatever you do, make sure you don't let your domain expire. Changes are coming in that will make this less of a problem but for now, get the whois on all your domains and note down in your diary three months, two months, one month, two weeks and one week before each expires. It'll only take you five minutes but could save you hours and hours of agony. There are reminder services available on the Net if you fancy.
• And if it has already expired? Contact your registrar immediately and ask for it back. You may be lucky, you may not. If you can't get it and it is due to go back out to the market, sign up to one of the programs that specialise in grabbing domains first as soon as they are made available. Best known in Snapnames.com, where you have a good chance of getting it back for $69 up front. Also sign up with ExpireFish.com and NameWinner.com to increase your chances. They only charge if you get the domain.
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And that's it. ®

Where the Hell is My website?
Part I: The nightmare and the US system (http://www.theregister.co.uk/content/6/29065.html)
Part 2: The UK system and what ICANN should do to be more like it (http://www.theregister.co.uk/content/6/29086.html).
Part 3: A practical guide to making sure you keep hold of your domain. (http://www.theregister.co.uk/content/6/29087.html)