This piece for Malachain collects the advice that holds up in practice.
Rules about walls and fittings do not stop a rented place from feeling personal. Textiles do most of the work: a large rug anchors a room, curtains that reach the floor make windows look taller, and cushions and throws add colour that leaves with you. Freestanding shelves and a tall plant fill corners that would otherwise need pictures on the wall.
For lighting, replace harsh overhead bulbs with warm ones and add two or three lamps at different heights. Removable hooks hold mirrors and light frames, and a strip of washable wallpaper behind a bed can be peeled off on moving day. Keep the original fittings in a labelled box so everything goes back exactly as it was when you hand over the keys.
The most useful step is usually the first one. Pick a single suggestion, try it for a couple of weeks, and then decide whether it is worth keeping. Small, steady improvements last longer than ambitious plans that are hard to maintain once everyday life gets busy again.
It also helps to talk the subject through with friends or colleagues who have tried something similar. Their experience points out the details that guides leave out: how long things really take, which costs were unexpected, and which shortcuts turned out to be a mistake.
There is more to learn on this subject, and the linked website goes into greater depth. Treat this article as an introduction rather than the final word, and check any important decision against up-to-date, reliable sources before relying on it.