1. If his/her room tends to be a disaster zone, talk to your child about what is working and what's not. Work with them to find a solution that they will use. Is the clutter mostly made up of clothing? Maybe they despise hangers! Consider shelves or hooks in the closet.
2. Talk about where clutter is ok and where it's not. As adults, many of us have a junk drawer. It's a place where clutter is allowed until it gets out of control (then we clean and organize it). Some children need a space like that too. Can they stash clutter in a drawer? Their closet? Is their entire room ok as long as communal areas (e.g., the living room) are kept clutter-free? Set reasonable expectations.
3. Rotate toys to help keep your child's interest. Keep some bins stored away of age-appropriate toys. When your child gets bored of the toys they have access to, pull out some from the stored bins. It's like Christmas all over again!
4. Contain it! Children work well with clear, labelled containers. If they can't see it, it probably won't get used. I keep puzzles in ziplock baggies in an open top basket and craft supplies in clear stacking drawers.
5. Keep a routine. Even though school is out (well, almost), keeping a somewhat regular daily routine during the summer (whenever possible) will help keep the peace. I don't mean that each hour has to be planned, I mean the sequence of events is generally kept the same. For example: wake up, breakfast, dress/get ready for the day, fun, lunch, educational/craft time, fun, dinner, read, bed.
Have a great summer!