If your work is hot, you get to set the price.
Does this happen in practice?
To pull this off, the contract would have to consider the future employer's needs. For example, my contract could say that while I am employed for one company, I can't be also employed at another one; especially if it is a competitor but also if it results in a negative impact in my performance: working under expectations. Make sure expectations are set beforehand. This applies to marriage as well
I would choose the freedom to work on my own company, as I want to further my own career and life beyond what is reasonable to work for someone else. I do want to be doing something in 5 to 10 years and nobody is going to complain much if I leave a company then. It is then in my best interest to add that clause.
However, if working on this separate enterprise is competing in resources or opportunity then it's out. This can be generalized as a simple antitrust clause: Work on whatever, but don't screw us
In law, things always get muddy
For example, if I have company property (computer) and I do some other work on it, some contracts generically stipulate that all intellectual property coming from that device is property of the company. I would be pissed about this. What if I tweet a million-dollar idea? Should I use gmail from my own phone? That would be ridiculous.
However, if I come up with something that negatively affects the market aspirations of the company, it is reasonable to assume that I am at fault. Again, if I can contribute to my employer with something I do on my own, it makes sense to share, especially if it's their equipment.
Contracts and the law in general have never been very deserving of my attention
I prefer using common sense and dealing only with people who have a healthy —or just similar— sense of trust. After all, the heart of the law is to protect from abuse, not to use it to abuse others. Unfortunately, the solution has been to write more law mostly to the benefit of those who invest their life trying to understand a little part of it.
When we stop communicating as human citizens and let behavior be ruled by the shroud of selfish intent, artificial means to protect our society emerge. Stupid laws. Contracts that nobody can read. Enforcers who get high on the power trips and are neither protecting nor serving. That stuff has to stop.
The law is silly because we shouldn't need it
If you think I'm saying it shouldn't be followed, then the law was made for people like you who miss the point.
Harsh but necessary.
I would love to hear ideas for simple clauses
Something like:Work on whatever, don't screw us and Work whenever, just get things done in time. From this, it is important to make sure that expectations about output are clear from the beginning and that all deadlines and work load is agreed to as reasonable beforehand.
I know it is not legally realistic, but I know that there are many startups that rely more on the trust of their employees than a perfectly drafted contract.