Crush Crush Guide

This section includes all of the things that you shouldn't spend diamonds on. They aren't worth a rare resource like diamonds or your hard earned cash.
Gifts and Dates
This is a waiting game. If you can't afford a gift or date for a girl, then you can wait a while until you can. If affording something is more than a week away, or simply more time than you're willing to wait, then it might be a good time to Soft Reset and increase your multiplier.
Outfits
If you are a F2P player, then there are far more worthwhile things to spend your diamonds on than outfits. If you are a paying player, then you should wait for diamond packs to go on sale and come with sets of outfits. This tends to happen a few times every year, usually at a time relevant to the outfits (Bikinis in summer, Holidays in winter, etc.).
Skip Reset
Soft Resetting is a necessary part of this game, there's no way to reasonably avoid it. You will never lose so much progress that it's worth 30 diamonds. Just soft reset, start from the beginning, and build your Prestige Multiplier for free.
Time Skips
Pro tip: You can exit the game, set your system's clock ahead a week, open the game again, and have another week's worth of progress. All for absolutely nothing. So don't waste your diamonds on doing the exact same thing within the game.
Time Blocks
Not being able to do everything you want to do at once can be frustrating. But prioritizing and putting things on the back burner is a skill that this game demands of you. Eventually, you get to a point where you have an abundance of Time Blocks and can do everything you need to do at once. You can be patient and get to this point for free. If you spend your diamonds on Time Blocks, you'll have an unnecessary excess down the road and fewer diamonds to spend.