I write this as a dating adult, not under the restraints of parental rules. That is my disclaimer here. These are not one-size-fits-all strategies, but these principles are what worked for my wife and I leading up to our marriage. These are insights gained from personal experience.
-Rely on the grace of God. I do not mean for this to be cliché. What I am saying is this: when it comes to seeking God, consistency outweighs intensity. Walk with Him consistently, and many things will be revealed as as you need them. Walk with Him consistently, and He will consistently guide your steps. How does that scripture go? ‘The peace that passes understanding will guard your hearts and minds.’ That is a big part of purity, in my experience: God’s grace and mercy in life situations and the manner in which those situations unfold. You will be less likely to end up in a situation that you shouldn’t have been in in the first place.
-Avoid idleness. Go for a walk if you want to spend time alone together. If you just sit together in a private place, it seems like there is more opportunity for things to escalate.
-Establish and honor real accountability structures. For example, as two people who each lived alone, we committed to not be alone together in either of our apartments. Aside from looking bad to anyone who might find out, we felt like it would just be too much temptation. So, if before or after a walk around the local parks she needed to use my bathroom or get some water from my kitchen, I would wait out in the entryway, and she would do the same for me. It may sound awkward, but really, it actually helped build and maintain a high level of respect for one another and for the principles that we had committed to.
-Communicate. Be honest and open with each other about personal convictions before you get into situations where they are tested. For example, many Christians believe not only in abstaining from sex, but also saving the first kiss for the wedding day. Communicate and reach an agreement about physical boundaries ahead of time. Communicate honestly about what situations you find to be most tempting. Communicate when something is messing you up and causing you to stumble, (even if only in your thoughts), and respect each of your expressed stumbling points.
-I felt way more sexually tempted in the last few weeks before our wedding day than at any other point in the relationship. I don’t know if this is true for everybody. Maybe it’s like a 4:30 effect. What I mean is that sometimes, the last half-hour of the day seems like the longest, and you say to yourself, “How am I going to make it?” If I knew it was going to be another six months, I could mentally adjust, but three more weeks, and I’m saying, “Dear Jesus, help me.”
-I wait because I love you. More than any type of moral principle, no matter how true it is, this reality, ‘because I love you’, has been the most motivating and compelling truth in practice. Your affection for the other person can be leveraged as a weapon for purity, not against it.