Depends on your visa. I ANAL etc, just spent a bit of time a while ago looking at the visa situation. Basically you will probably get away with a B-1 visa for your three month run very simply (electronic registration online and ~$15). It's a bit questionable as the work your doing could technically get you in hot water though deal signing and seminars are fine.

After those three months you will have to leave, you can come back on the same type of visa (though it is more likely you are breaching it) but you'll likely be asked more serious questions and if a border agent doesn't like you then you could be out of luck and barred from re-entry.

One of the key things to remember is the penalties for getting caught breaking any visa conditions could essentially get you barred from reentry for life.

So after that first three months you'll probably want a visa that line up with the fact that you're now effectively working in the US.

H-1B, which will require your startup to be up and running and look reputable and they are in short supply.

O, which will require you to show your a respected top of your field type person (i.e. articles written about you, speaking engagements).

E-1, your company must be non-us (which isn't ideal for YC) and a substantial volume (and over 50% of the international trade) of trade must occur in the US.

E-2, 50%+ of the business must be owned by investors in your country (you and your co-founders). Possibly your best bet but you'll need a substantial amount of your wealth (cash,IP (which might effect how you work for your company to set up IP transfer which might not be agreeable with other investors)) committed to the business (100K+) in a non-revocable fashion. Also if the business fails your visa is no longer valid. Risky but probably the most achievable in terms of startups (with regards to getting back in the country doing it quickly and with a high degree of certainty).

L-1 this will require you to have an office in your own country as well, you'll effectively transfer yourself between the two offices. Keep in mind that immigration aren't robots and you'll need to show your not just playing a shell game.

If your canadian(mexican too I believe) TN, basically with an offer of employment from a US company you can work there for 3 years. They'll need supporting details and you'd do well to have the company at a point were you can technically get fired to be credible (i.e. board of directors/you don't own 51% of the company).

Edit: Also visas have intent. i.e. Plan to immigrate or not. If you have a visa with no intent to immigrate and you look like you're planning to immigrate then you can be found to have broken your visa conditions and barred reentry/kicked out. So if you grab a work visa that does not have intent to stay then apply for a visa that only has intent to stay your current visa can be invalidated, you may have to wait until your current visa expire before applying for a more appropriate one.

The go to advice is find an imigration lawyer, they'll have a better idea than you or me on how to set this up.

There are also more options that may open up as your business grows but if you're at the point of considering them you are at the point were you'll have the resources and responsibility to get lawyers to do the majority of the leg work finding/explaining your options.

There may be better news on the horizon:

http://startupvisa.com/