1. Store all meaningless decorations (non-premium AND non-limited time)
2. Move all buildings to a corner - only store meaningless ones - colored houses, Krusty Burger (except original), Kwik-E-Mart (except original) and Gulp N Blow (only keep original)
3. Next pick a different but accessible corner and work from there. Move buildings to new locations/areas, place roads/pavement/rivers.
4. Once all buildings and premium/limited time items are placed, open up that inventory and take out the other stuff.
I find it easiest this way. There used to be issues losing stuff out of the inventory - which is why I say don't store premium/limited time decorations. Premiums will still help your multiplier, so another bonus to leave those out.