If you inject insulin regularly, you will need to vary the areas of skin you inject your insulin into to ensure your insulin gets absorbed consistently.
Also, by rotating your injection sites, you can avoid developing stiffer, lumpy skin helping you to feel happier.
Which areas can be injected into?
Ideal areas to inject into are parts of the body with a decent layer of fat.
The belly, upper arms, thighs and buttocks are commonly used. Note that some parts of the body absorb insulin quicker than other parts.
The quickest area to be absorbed from is the belly, followed by the upper arm, then the thighs and lastly the buttocks. 
Charity Diabetes UK warn that the arm may not always be a suitable injection for people with less body fat  as there is a greater chance of injecting into a muscle which could lead to hypoglycemia
Injecting in the same general area for the same type of meal
To have consistent absorption of insulin, it’s recommended to inject in the same general area of the body for the same type of meal.
For example, for breakfast it may be a good idea to inject short term or bolus insulin into the belly. For long acting or basal insulin, it might be beneficial to inject into the legs or buttocks.
You can discuss which injection site is best for each meal with your health team.
Rotating your injection sites
If you inject regularly, it’s recommended to ‘rotate’ your injection sites. Rotating your injection site means using a different spot to inject into to prevent injecting into the same place each time.
For example, if you injected into the left side of your belly yesterday at breakfast , you could inject into the right side of your belly for the next day’s breakfast.
You’ll soon need to inject into the left side of your belly again, but when you do, pick a different part of that side of the belly to inject into. It can help to imagine a grid drawn onto your body and you’d aim to pick a different square to inject into with each new injection.
After some time, you’d need to inject into the first square you chose but that should usually have fully healed by that time. 
Why do I need to rotate my injection sites?
If you inject in the same place too often, your skin may start to become stiffer, often described as being ‘lumpy’. The official term for this is lipohypertrophy and is the result of extra fat being produced at that part of the body.
This poses two potential problems. The first is that injecting into lumpy skin can mean that insulin doesn’t get absorbed as consistently as normal. The second problem being that lumpy skin can look less attractive.
At clinic visits, your health team will usually check the skin at your injections sites to ensure you’re not getting lumpy skin. If you are, they can help to advise you how to prevent it.