You're short of money; perhaps you're saving towards buying some back to school or Christmas hardware, or you need the cash to put towards an upgrade.

Either way, selling your old smartphone is a great way to raise funds. That is, as long as you do it right.

It's easy to snap a photo of your old phone and upload it to eBay with a price and hope for the best. But for the best results, you need to treat the sale as if you were selling a house or car. Take the sale seriously, treat the potential buyers with respect, and make sure your phone (and the same tips apply to tablets) looks good.

Get Your Timing Right

Selling is about timing -- everything has its peak value.

The best price you get for your phone will typically be within the first year. Take the iPhone; updated annually, the optimum time to sell is about a month before a new version is announced (typically September). The same thinking can be applied to the Samsung Galaxy S series - you'd get more money selling the current model, rather than "the previous model" after the new version is announced.

With regards to the iPhone, devices that can be upgraded to the latest version of iOS will always generate a higher sale price than the older handsets. So research what operating system version your device is capable of running, regardless of platform. If it can run the latest update, then you may safely ask for slightly more money.

But how you sell is also dependent on timing. If you plan to sell via eBay (assuming you still trust the service), timing your auction to close on a Saturday or Sunday can help to push the sale price up (useful for paying for those eBay fees).

Protect Your Device

You set the resell price of your smartphone from the day you start using it. Cases, screen protectors (which aren't so difficult to apply correctly) and careful treatment of the device will ensure that it is in good condition when you come to sell.

Broken displays can be replaced, while scratches on the body can be avoided with cases to keep up the resale value. If you find any scratches, ensure that these and information on other blemishes are available for potential buyers to review.

While this might be the difference between a sale and a buyer walking away, it is preferable that they walk away now rather than come back later and demand a refund.

Bundle Some Useful Extras

What will sell your phone? Can anything make your sale more attractive than LadiesMan217's?

Always include the original packaging, charger, SIM adaptor and SIM slot key. This shows that you cared enough to keep them, and increases your device's resale value.

Various low-cost extras can be added to sell your old smartphone as a bundle. Think screen protectors, cases, dashboard holders, wireless chargers (where compatible) and desktop docks. By selling your old device along with all of the extras that a buyer could want, you save them from shopping around for the same items. It's a win-win situation that will reap rewards.

SIM-Unlock The Device

Wherever possible, you should always SIM unlock your smartphone (potentially tricky for iPhones). This isn't the same as unlocking the OS (known as "Jailbreak" on iPhones and "rooting" on Android), but rather refers to breaking the link between your device and the network.

Without taking this step, you'll be limited to selling your phone to people on the same network. In the USA, this can also mean that you're restricted by location. Why sell to people in only four states when you could unlock and sell to all 50?

Unlocking can be free, but usually costs around $20. This is a great way to generate a higher sale price, however – as soon as the potential buyer learns that they can use the phone on their network (or any that they desire) they will be immediately prepared to pay extra to get the device.

Use Amazon's Trade-in Service

Amazon Trade-In isn't as well-known as some of its other services, but a quick visit to the website will show you that perhaps the most popular trade-in items are smartphones.

All you need to do is select the phone you're selling, set the condition details option, accept the price and send the phone off to Amazon. At this point, the device will be assessed, and you'll either get the higher price or a lower figure based on the phone's state.

Payment is in the form of Amazon eGift Cards, so useful if you make a lot of Amazon purchases, but less so if that isn't how you would like to be paid.

(Note that Amazon Trade-In has closed in the UK as of August 2015.)

Sell Your Phone Locally

Using online marketplaces like eBay, Amazon, Gumtree and others is a good way of selling your phone to the widest possible market.

However, it may be simpler and more financially rewarding to sell locally. Placing an advert in a local newspaper or one of the many "for sale" groups on Facebook is a good start. You can also use community noticeboards, and if you have a local shop that accepts "for sale" cards in the window or on a dedicated board, then it makes sense to take this route.

At college or university? Spread the word among your fellow students, using noticeboards, regularly-visited campus websites and magazines, taking care, of course, to keep the device safe.

Have you sold your smartphone successfully? Did you make a surprising amount of cash? Perhaps you were lucky enough to own a rare model? Tell us your tips and secrets in the comments.