Regarding cheat codes: they're usually present in "sim date" games. If you're unfamiliar with these stat-driven Flash games, then try Pacthesis' games at
http://pacthesis.deviantart.com/gallery/27555536 . There are more on sites such as Newgrounds, but I recommend the ones written by Pacthesis because they have decent writing, characterization, and sometimes a fair amount of plot. ...Anyway, in a decent sim date game, the player will be given at least one opportunity to enter a secret password. It will increase stats and sometimes bonus content.
It's your decision if you want the player to win easily. You can let the player choose difficulty (from challenging simulation to simple visual novel). Flowerthief did this with the freeware game Heartache 101. You can let the player bypass puzzles. You can implement a "New Game Plus" (named after a feature in Chrono Trigger) mode if you want, which lets the player start from the beginning with excellent stats, items, etc.
There are strategies for succeeding with the Tokimeki Memorial games. After browsing GameFAQs pages, I see they have a couple trivial cheats, but nothing game-breaking. I didn't search much, but I couldn't find any outright cheats for LovePlus, Pia Carrot 1, or Otometeki.
KimiYoriBaka wrote:
some things I've been considering for my own dating sim
1. having the heroines find it question the player for trying to be too nice to them after already going out with someone else.
2. having the heroines do evil things to the player if the player tries having an actual date with multiple of them
3. having the player get arrested for too much stalking...

These are all rather good ideas, and they could all work in romance games. If you like these concepts, use them. The first idea that KimiYoriBaka suggested is somewhat relevant in Persona 4, though the game doesn't exactly emphasize simulation. The second idea is similar to the "bomb" system in the Tokimemo games. (Read the FAQs if you're unfamiliar. It's basically a measure of an NPC's anger at being ignored. In the Girls Side games, the main character can also become a rival of her fellow girls.)