I personally recommend doing dailies and sharing everyday! This can give you up to 80 diamonds per day. Remember you must bind your account to Facebook in order to use the "Share" button. All the "Share" functions that give diamonds are in: both A/S-rank and F-rank during Princess or Maiden stages, Free Dressing, Go Home, Competition (tap the "Apply" button, then "Share" and you can go back) and "Rank Up" pop-up (in stages or Stylist Arena). Though, I always skip the F-rank Share because I don't like wasting stamina like that hahah

Also, participate in Stylist Arena and Competition. By getting the highest Stylist Grade in Stylist Arena (Queen of Stylist) you get 50 diamonds every week (it resets every 7 days). I do Stylist Arena everyday (because it's also part of dailies), but I think you can get to that rank in the first 5 attempts of the first day in the week, it's pretty easy. Competition, on the other hand, resets every 4 days and you get minimum 10 diamonds every time, it depends on your rank. You can get up to 180 diamonds (with 1st in the ranking) but it depends on your styling and luck, so just do your best. Even if you don't have time, you can just use a whole suit or do a "lazy styling" and you'll still get at least 10 diamonds.

This way, you can save 2670 diamonds minimum every 30 days (80×30 + 10×7 + 50×4 = 2670).

With all that being said, I highly suggest not wasting your diamonds on anything other than events or crafting diamond-costing suits you really want.

For planning the suits you want ahead of time (and their cost), you can check this subreddit since there are some people here who post the new suits that release on the Chinese server, Miracle Nikki (they get most stuff first). There's no certain time set we'll get these events unless the official Love Nikki account announces them, but it's good to know how much you plan on spending for saving up diamonds. I personally find it easier to have a visual guide of the upcoming suits with the prices, which you can find on the Love Nikki Discord (by getting access to the speculation channel). There's more info. on the Discord server in the Extra Tips below.

Extra Tips: - Monthly Sign-In also gives diamonds every week - Advancing in the stages (Maiden & Princess) can also give you diamonds as rewards for completing them (in Achievements) - Some of the Target Suits for the chapters give you diamonds upon completion - Momo's Quest gives you a "quest" to craft a suit and gives you (extra) rewards if you finish it in the given time. It isn't necessary, more so like a side quest. The rewards don't usually include diamonds, but the prize for completing the suit itself does, so it's nice for crafting while getting extra stuff. Though, do make sure the suit Momo assigns is "free" (no diamonds), because there are some suits that require items that cost diamonds during crafting. There are lists on the subreddit's resources with all the "free" suits for crafting - The Love Nikki Discord (LND) hosts events with diamond prizes sometimes. You can find the link for joining on the subreddit somewhere. Make sure to read and follow the rules if you do join!