IP is a sticky issue with most contracts involving creation of a product (especially R&D). My uncle gave me some valuable lessons that I have mostly followed since then(enough money can be an exception ;-)

First of all, spell out in concrete what is claimable IP. When he was working for RCA he created some very valuable "items". They are quite commonly used today. The items were completely unrelated to his work, but because of his IP contract at the time, he had no rights to them. He renegotiated his contract, and got it spelled out where any value added IP beyond the scope of his current employment was still the company's, but now he was compensated with bonuses (some of them quite nice).

His next position, he got the contract to read that all IP not directly related to his work was his. The company had to purchase it to own it. Now, he contracts all around the world, using the same type of language in his contracts. He has an office in downtown Boston and travels to 6 to 8 countries per year to consult. All paid for.

He has told me that he does not think he could have ever achieved the level of independence (and choice) that he has if he had not negotiated the IP clauses the way he did. He got payment for his extra work, but more importantly, he got his name on those pieces of IP, and everyone in that industry now knows him for those pieces of work. They hold international conferences up for him if his plane is late. He says the money was good, but the recognition for what he actually did is what really was important.

Moral of his story. Get the IP spelled out well. Then work your arse off developing tangential solutions in your spare time. Gain the recognition by getting your name on the solutions you create and insuring that your name will always be there. It is your future that companies steal from you when they "appropriate" your IP. The income is inconsequential in most cases compared to the doors that open when your name is on the ticket.