In my 15+ years in IT consulting and managed services, I've seen people make a lot of mistakes. So I’m listing some of these in the hopes that you won’t make them yourself:

Auto-renewing contracts

Service providers, from managed services to Internet and phone companies all want to keep their customers. Unfortunately, some of them do this with auto-renew contracts that ensure successive terms continue after the initial term expires. This is very common with Internet and voice circuits where a customer signs a two-year agreement not realizing that when it expires, unless the customer specifically notifies the provider to cancel, it will automatically renew for another two years. Whenever signing any contract ensure that at the end of the term it doesn’t renew, or it auto-renews on a month-to-month basis.

Domain name trickery

I've seen dozens of letters (both email and postal) from companies attempting to get my clients to either move their domain name to a new registrar or to purchase international versions of their domain name (i.e., tabush.com.cn, tabush.cn, etc.). The letters are cleverly crafted to look like official notices which must be signed and sent back (with payment of course). Just like you would forward any tax-related mail to your CPA, you should forward any domain notices to your IT service provider before signing anything.

Faith in the SLA 

Most IT and telecom service providers have an SLA (Service Level Agreement) in place with their customers. The SLA typically entitles the customer to monetary compensation in the event of a service outage or problem. Some providers try to use the SLA as a security blanket, reassuring their customers that because they have an SLA there’s nothing to worry about. But have you ever actually read an SLA? They can be misleading. For example, a datacenter which provides mission-critical services offers "up to $100 per hour of downtime" as compensation which must be requested by the customer. If a company's system is down for three hours on a business day, do you think they really care about ore have time to file the proper requests for this $300? Providers count on the fact that they won’t. In my mind, an SLA that you can trust is one that allows a customer to terminate a contract if the provider can't provide proper service. This forces the provider to keep the customer happy, and this is how Tabush writes our SLAs.

Buying Cheap PCs 

A typical business computer, including proper software, warranty, and quality components, costs around $1,000. Some PC manufacturers like to advertise a PC for a low price like $399. Companies buy these PCs and hand them over to their IT provider to install, thinking they just saved $600, when in reality they haven't. In fact, the $399 PC will probably end up costing them more than the $1,000 PC when all is said and done. Why? For one, $399 PCs include a Home Operating System (these days its Windows 8 and not a tried-and-true OS like Windows 7 Professional). It won't include Microsoft Office, instead it will include a ton of "free" programs (typically trial versions that vendors pay to have included) and then your IT people have to spend time uninstalling these “freebies.” And, you’ll only have a 90-day or 1-year warranty with tech support provided by the cheapest and least-qualified people from some foreign country you've probably never heard of (strangely enough these people will have western first names like John, Peter and Steve). The components of these PCs are of the cheapest quality and will need updating so often that there’ll be no standardization throughout your company. 
On the other hand, computer lines like Dell's Optiplex series (which is what we’ve been using for years) are of superior quality, use standardized and consistent components, come with the right software pre-installed, and have excellent tech support. They’re well worth the money.

Redundancy Isn’t a Backup 

Most people confuse these terms. Backups are snapshots of your system or data that’s captured at a specific point in time, and is (hopefully) stored somewhere safe. Redundancy provides multiple systems to prevent downtime in case one fails. The most common form of redundancy in IT is RAID (Redundant Array of Independent Disks) — a form of mirroring data across multiple hard drives. This is important because hard drives do fail, and RAID protects you from this. But RAID is a live mirror or copy of your data, so if you delete a file or get a virus, that is also redundant. Plus, RAID doesn’t let you go back in time to recover data.

Note: Apple's iCloud backup for iDevices is nice if you lose your phone or it’s damaged, as it provides an automatic backup, but it doesn’t let you go past your most recent backup. So if your phone backs up each night when plugged into your computer or on Wi-Fi, you can't retrieve something that was deleted prior to that backup. For historical backups, you need to do a backup to your computer via iTunes.

Backups 

While we're on the topic of backups the reality is that they are one of the lowest priority items on most companies’ IT to-do list; and down the line these companies end up paying the price when they suffer a data loss that can't be restored. Your data is your business. You’re paying people to sit at computers and essentially create data all day long, be it orders in an Enterprise Resource Planning (ERP) system, software code, sales materials or product designs. It’s critical that you protect this data. There are many forms of backup and many levels of service. Get professional advice, pick the one that is right for you, provides proper storage and security of your data, can be monitored regularly, and is within your budget. Then test the backups regularly. And remember, you may still need redundancy.

Not planning for downtime 

Despite all your backups, redundancies, monitoring of systems, audits, and however much you try to prevent downtime, it’s a part of IT and therefore a part of life. Even Fortune 500's have outages from time to time (including Amazon, Chase and Salesforce.com). All businesses should invest in preventing downtime, but they should also be prepared to deal with it. With the right IT downtime solutions you should be able to work offline, take a laptop from work to an Internet café and continue your business, or hold team-strategy meetings online while your IT provider works on resolving the issue. You don’t need to just sit around wasting time.

Conclusion

So there you have it. Hopefully, you’ll take away at least one or two tips from this post. And if you’ve made some of these mistakes in the past, don't be too hard on yourself — they are common mistakes after all.