The optimal financial structure is one that minimizes dilution.  If the business involves some collateral, then you can use debt financing for part of your needs, minimizing dilution.  The debt investors will most likely insist on a layer of equity, however.

If you cannot raise debt financing at all, or must raise both debt and equity, the goal in theory is to raise only enough equity to get you to your first valuation event, which presumably will enable you to raise your second round of financing at a higher valuation, again, minimizing dilution.

This is a tricky issue, however.  On the one hand, it is nice to keep as much of the company as you can for yourself.   On the other hand, if you miscalculate how much money it will take to get to your first valuation event, the company could end up going under.  You will also run the risk of having to raise round after round of financing, leaving little time to manage the business.  Maximizing your percentage ownership of a failing business is cutting off your nose to spite your face.

So, in general, it is good err on the side of raising a bit more money than you think you will need to reach your first valuation event.

The other decision you should make, once you have a general idea of your financing plans, is choice of corporate entity.  For years, it was considered a foregone conclusion that one should form a Delaware C-Corp based on the idea that that was the form of entity with which investors would be comfortable.  That advice no longer holds. The tax advantages and flexibility provided by LLCs are now winning the day, for good reason.

However, it may turn off some investors if you get too cute in taking advantage of the flexibility of LLCs in your operating agreement.  The tax advantages of LLCs are all well and good, but don't go too far in creating multiple classes of shares and keeping every bit of control for yourself regardless of how much money you raise.