Know your PhD partner. This is a general dating rule that applies to all situations, but it's particularly important when dealing with a graduate student who immerses her or his life in one area of study. PhD students spend hours a week in a lab, or reading, or talking and thinking about a few very particular topics. Knowing what their lifestyle requires is an important part of knowing how to navigate it.
Know what your PhD partner is studying. You don't have to get a PhD yourself to know a bit about what your date studies. The mating habits of the Gambian epauletted bats of Africa? The effect of late modernity of immigration policies in France? What? It may seem daunting, but taking an interest in what your date is studying will show her that you understand how important the subject is to her.
Make time on a regular basis, if only a bit of it. Believe it or not, PhD students are busy. It's not all macaroni and cheese and video games in your underwear. Classes, teaching, researching, writing are all part of the PhD lifestyle, and they take time, lots of time. And you are busy, too. Finding time, even if it's dinner, a walk, or a brief phone chat a few times a week, goes a long way to showing interest and commitment to keeping the relationship alive. Occasional text messages also help achieve this goal, but keep them short; your PhD partner has enough to read already.
Make vacations count. A few hours a week may keep a budding relationship alive, but it won't work in the long term. The key here is planning. Sit down and plan your upcoming free time together. PhD students often have irregular schedules, but are often able to work out reasonable vacation time between terms, during the summer, and over spring break. Try to match up your vacation time with your partner's, and take off together. It doesn't have to be a whirlwind trip to the Amalfi Coast. A few nights at a quiet, local bed and breakfast can do the trick by offering some time for well-earned relaxation. A ski trip may be good, too; it's quite possible that your PhD partner needs a bit more vitamin D.
Meet the faculty and friends. If your partner is up for it, make friends with his department. PhD students tend to become fairly close with their faculty, support staff, and other students, since these tend to be small and closely integrated communities. Department members end up doing a lot together, such as after-hours drinks, trips, holiday parties, book clubs, weekend pick-up sports games, and all kinds of other things. If your partner is comfortable with it, try to be a part of some of these events. You don't need to attend everything, but showing some interest in a few events from time to time will help you integrate yourself a bit into the PhD lifestyle and community.
Ask for, and expect, some reciprocation. Dating a PhD student takes work, but it's only going to work if he is also willing to put in the time and effort. Your PhD partner needs to be interested in your life, work, and community as well. If your relationship is going to work, there needs to be mutual interest and respect for both of your lifestyles. Things won't always work perfectly, and sometimes one partner may need to put more work in, but, in the long run, a general balance should be expected, reached, and maintained.