How can I make a flat homely?

I do like a houseplant or two around the place.  

If you want pictures/posters then buy a free standing bookshelf, take out the top shelf and mount a picture in there. Use the other shelves for, erm, books and things.

Try floor lights pointed at the walls. They can be hidden behind chairs and other furniture but help light the room without being top-down bleak 'look how bare I am' and instead break up the expanse of the wall themselves.

If you have wooden floors, add a rug.

Instead of mounting pictures on a wall, set up a cheap projector to display a slideshow on it.

Paint a mural on the wall. No holes and you pre-meet the requirement to paint before moving out. (Ok, not this one).

Just wait. Over time your home will reflect the person living in it and whether others think it's bare, untidy, sterile or dull, it's your home and you'll be used to it. Sod them, it's what you're happy with.

Get a cat. Cats make any home homely. Better yet, get three. Three is a good number for cats.

Find an interior design discussion forum and ask them how someone with no DIY skills, constraints on modifying the property, financial constraints and is on the Autism Spectrum can turn a house into a home. Ask nicely, ignore the idiots that don't take up the challenge and thank the others for their helpful suggestions to encourage more. I suspect the experts can add to the good suggestions others have already made here.

I've tried those. They just die and/or get knocked over spilling dirt all over the place.

Thank you for all the advice. There are some good tips, I appreciate it.

And also lamps. Having lamps makes a room feel warm and homely instead of having the big light on.

Houseplants! Of all shapes and sizes.

Try colourful cushions, soft textured rugs, coloured bowls, bright bean bags and modern lampshades.

Plastic said:

There's also Command strips that magically hold things to the wall with no holes banged in and the come off easily afterwards leaving no damage.

Which bit of that involves banging holes in walls?

But I told you I can't do that. Why ignore what I said?

I do a lot of wall repairs and I've found that the Wickes powdered filler that you mix with a little water to be best - easiest to apply and easiest to sand smooth.  A splodge of paint makes it perfect.

There's also Command strips that magically hold things to the wall with no holes banged in and the come off easily afterwards leaving no damage.

I don't think it's at all unreasonable to put pictures and paintings on the walls.  Most landlords expect a bit of natural wear and tear, and unless they stipulate 'no holes to be made in walls', then I'd go ahead and put pictures up.  I've actually got a wall-mounted bookshelf, which required drilling six deep holes in the wall.  It sounds like major damage - but whenever I've taken it down in the past to move, it's usually just needed a small amount of filler (you can buy tubes or tubs of the ready-mixed stuff in pound shops), then a blob of paint on top when it's dried.  I'm no DIYer, either, but that kind of thing is easy enough even for me.  Personally, I find it's those very things - pictures, prints, mirrors, photographs - that make a place seem more homely and less bare.

I know it's not everyone's preference, but I like to be surrounded by books, so I have a couple of large bookcases full of books.  I tend not to like modern stuff so much, so I haunt charity shops for little things - candlesticks, vases, bowls - that I can put here and there for homely touches.  My sofa came from a charity shop.  So did my armchair and coffee tables.  Many people would look at my living room and see it as very cosy and homely, but it didn't cost much to do and it was relatively easy to find the stuff locally.

I live I a housing association property, and have decorated how I want, if you are planning on staying in the property meaning it is not short term accommodation then you can decorate how you like. 

Most housing associations use that term in tenancies but it really applies to damage not small things like holes from picture hooks or curtain rails. It can be misleading.

Have you tried different types or colour curtains/blinds? They can really change a room