Congratulations! You've met your perfect investment partner. You've both engaged one another and even though it may not be 'til death do us part, you should both strive to make sure that it's for richer and not for poorer!
Any successful relationship, whether business or personal, is always worth more than the sum of its parts. Once you've made that commitment to your partner, then it's in both your interests to become stronger and build your future together. So, in order to fulfill your obligations, you must ensure that you understand where your own responsibilities lie. Openly communicate with your investor so that both sides understand the other's expectations; and then live up to your end of that contract.
As I hinted before, you are going to have to take on new roles in order to retain a larger piece of your pie. In addition to the bigger-picture business planning and marketing strategies, you'll also have to make sure you don't miss all the smaller scale frontline tasks that happen "magically" when working under the wings of a traditional publisher.
Don't forget details like registering trademarks, securing website domains and social media profiles, setting up community forums, and moderation, public relations, and much more. All these things take time and you should never assume that you'll be able to "do it on the fly."
Plan your business -- and then execute that plan. If you're thinking that "jumping through these hoops" isn't your job because you're the developer, then think again. You can't take it for granted that your investor will "handle" these things for you; it's not his job.
Perhaps the biggest commitment you've made is to deliver on your creative promises. Don't think that having a personal relationship with an investor will mean they'll cut you some slack if you miss an occasional milestone. If anything, hardcore investors can be even less forgiving than your average publisher.
...and you've created a contingency plan for when things go wrong, right? With the best intentions in the world, there'll always be curveballs thrown at you, so make sure you have contingencies in place and come to me with solutions instead of problems. Even the strongest partnerships have rough patches but when they arise, communication with your partner will dictate whether or not you are able to save, or be forced to terminate, your relationship.
Imagine you've had a major technical problem and you're going to miss your milestone. The investor might be sympathetic, but he won't necessarily be interested in the details, or in hearing excuses. As far as he's concerned, he delivered on his promise but now his money is being tied up or, even worse, diluted. So what do you do?
First and foremost -- you be honest! You bring your investor up to speed and you do it without delay. You don't hide, you don't blame anyone else, and you don't whine about how terrible everything is. You come to them prepared and you explain how you're going to correct the problem and get everything back on track. And then you do just that! Your investor may be annoyed, but they'll respect your integrity and you won't jeopardize their trust in you. I can't stress this enough.
Even if things go smoothly during development you'll still need to be constantly aware of what's happening in the broader business environment. You must be able to adapt your plan on the fly and never get caught out by an "unexpected" OS update, a new platform or business model. Anticipate the trends and use them as opportunities to make your product better. I want to see you being proactive and taking the time to understand what I, as an investor, need to know.
As in any relationship, it won't always be smooth sailing, but if you remain faithful and committed then, in return, you'll have the unconditional support of your investor; as well as having gained a powerful ally who potentially could have an interest to become a more permanent part of your future. While investment never comes easy, the potential benefits of choosing the right partner can more than make up for that effort.
Remember: A good investor can't ignore genius -- and neither will they reject something of immense value!