Just make sure that you're with a reliable registrar (good ones will hold a domain for a short grace period after the actual expiry — their own "registrar's renewal expiry" may be a week or two ahead of the root TLD's expiry date — although they may display an ad page instead of the actual site after a week or so). Renew your own domain(s) ahead of the expiry date; you will usually be notified that renewal is required at whatever address you have as the whois entry (and if your registrar is hiding the whois from the public, THEY still know what it is). It's imperative, then, that you keep the whois data up to date if you change you business name, addresss or notification address. That will make sure that there is no window of opportunity for a grabber to register your domain unless you are letting it expire deliberately.