To prevent always the same items from being purchased, I use the Arkham Horror rule: draw the top three cards from the item pile and you must buy exactly one of them (if you can afford it). The other items are discarded. The event “If I could just reach…” allows you also to choose from the item discard pile. If you do, you might purchase an item for half the price (rounded up).
I ususally play with only one expanion board. To give the heroes a better reason to visit the new board, I came up with the following rule that applies as soon as the Shadow Track marker reaches the second stage: If, at the beginning of the mystery phase, there should be ever more than eight investigation but no hero on one board, the shadow track marker moves one space ahead.