Making a teen a partner in the redesign can be fun. Let them have complete leeway. It gives them the motivation to keep it clean.
A completely new set of furniture is not necessary, but a few items may be - a bigger bed, for instance.
Another piece to consider investing in is a chest of drawers. Go for classic lines, not something too youthful.
A vintage chest of drawers is a good idea. Teens have different priorities from little kids.
They don't care much anymore about vast floor space to build Lego cities.
Turn over some of that real estate to a couch and an overstuffed armchair for reading, and to creating a hangout/study nook worthy of having friends over, even if it means going for a laptop desk for homework over a freestanding one.
A lamp can be made new with a funky fabric shade, and many home improvement stores carry beaded chandeliers in a variety of colours.
There's no better way to let a tween or teen take control of his or her space than putting them in charge of palette, but be prepared to accept whatever crazy colour scheme they come up with.
Besides walls, linens can add a blast of colour and texture. Wall art is huge to many teens.
How about a collection of colourful album covers from the 60s, a surfer scene, or framed memorabilia such as photos of friends and family mixed with your child's own artwork?
Dress windows in wild designs and colours. Curtains come funky and cheap these days, or buy inexpensive shades they can paint themselves.
The idea is to cede some control, but not get thrown under the bus altogether on style, design and cost.
Don't forget to mount a full-length mirror on the back of a closet door. It is a cheap, easy way to more quickly extract a teen girl from a bathroom.