As has been said, the easy part of the answer is a secondary MX pointer. The hard parts are generally how many (it depends) and where to host any secondary SMTP servers.
If you are located in an area which is prone to Internet outages or your hosting provider is prone to outages (still happens), then arranging for secondary SMTP hosting at a stable location is "A good thing to do"tm.
If you only have one Internet link, then moving up your provider chain to the most stable provider (generally your service provider) would be the best action. This also allows for email store-and-forward when your site is offline due to issues beyond your control, (Nature, utilities, construction, etc.). Availability and cost vary by provider, but the service is generally a negotiable item.
If the secondary SMTP host is configured to accept all mail and only forward to you, then the issue of SPAM and Virus checking becomes a single point, but this is rarely the case and you would need to have this as part of your secondary agreement so your domain is not flagged as promoting due to inaction.
Multiple Internet providers allow a much more interesting set of solutions to this question.