1) Organization. You'll be on the road a lot and so will only have a day or two together, so make sure you have nice stuff planned on weekends beyond just drinking or sitting at home and watching movies. The nice thing about consulting is you make loads of money and don't spend it, so take the opportunity to explore your city. Weekend travel and flying them out to you, if on a travel project, is great for this.
2) Expectations on communications. Do you talk on the phone every night before bed? Every second night? Do you text during the day, or no? Make sure both of you are aligned on that
3) Going offline. Both of you will tend toward checking your phone constantly for email or talking about your project. Don't. Or at least discuss how to do that ahead of time. Phones on the table at restaurants, yes or no? While watching TV? When you meet up, consider having the first 20 minutes be work-related, then not.
4) Keeping healthy. This is more than just relationship, but it takes conscious effort to exercise, sleep enough, and eat relatively healthy when you start consulting. If you're stressed, tired, and aren't eating well or exercising, your relationship will bear the brunt of this. Talk to your colleagues how they keep healthy, and see what you can learn from that. Maybe that's a weekly class with your partner, maybe that's skipping work team dinners or Friday drinks now and then so get some sleep.