A dog or cat, a house plant, a worm farm... whatever it is, when you spend time alone it's encouraging to have something to think about that isn't yourself. Otherwise it's easy to become too self-involved.
Keeping something that needs you to maintain it every day has a good grounding effect, especially when it's something that forces you outside of your house.
BE PRODUCTIVE
It's all well and good to spend your alone time on the couch watching movies. I like to allow myself a bit of this, but it's important for my mental health to feel productive and to achieve every day, no matter how small or trivial the task.
You can clean your windows or make a plan to take over the world; whatever gives you a sense of accomplishment makes the alone time feel worthwhile.
DON'T GUAGE YOUR HAPPINESS LEVELS ON SOCIAL CONTACT
I've spent many-a-weekend feeling defeated because I have not been terribly social with friends. Days could pass and I'd be left in a slump if I'd failed to see my mates; as if such contact validated my experience by satiating a need for human contact.
Changing my thinking around this was key in properly utilising my alone time. I put my feelers out for social contact earlier in the week, and if it didn't happen, people cancelled, or another spanner was thrown in the works, I saw it as an opportunity to do something else alone. Sitting around moping because you have no social life serves nobody well.
REMEMBER HOW TO APPRECIATE SOMEBODY
When you're faced with alone time, you should be reminded how much you enjoy having special people (romantic and platonic) in your life.
If you're never alone, you never have to experience the feeling of pining for someone's company. This is an emotional state that significantly pays off when you finally do see them again.
Alone time teaches you how to appreciate the people around you when they ARE physically there, and not take them for granted.
SLEEP DIAGONALLY
This one seems small but brings me great joy. No matter the size of your bed, sleeping diagonally is one thing you can never do with another person in it. It's always the first thing I revel in when left alone.
Really, truly, stretching out in bed and taking over - especially on days you have the luxury of a lie-in - makes you really appreciate how comfortable your own personal space can be.