1) Mix or match: My favorite collages have various frames and mats. But matching mats and frames can look nice too.
2) Spacing: The trick to getting it to look right is to space the pictures fairly close together. You want them no closer than an inch of each other but also no further than about 3 inches.
3) Arrange on the floor first: I always lay them out on the floor first to see what placements I like together. Big pictures can serve as nice anchors, surrounded by smaller pictures. Keep moving them around until you like it.
4) Balance vs. symmetry: Try for balance from top to bottom and side to side, but not symmetry. Let the arrangement be kind of organic and not too predictable.
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