Having said that, if you are a member of any frequent guest programs, sometimes certain classes of membership allow you to get early check-in.  Other high-end hotels will sometimes accommodate you if you mention that you need an early check-in, as well.  I know that if you book certain hotels through American Express' Platinum travel service, then you can get that.

Other than that, it's really down to you being able to pick a hotel that is not as booked and which will have rooms available.

Sometimes hotels will also accommodate you for an upcharge, or if you book a type of room which is less frequently rented (handicap-accessible or a higher class of room, such as a suite).

I am pretty sure you want to check in early so you can get into the room and do what you have to do.  On some occasions, I've been able to check in my luggage and leave it with the baggage clerks, go off to other business and return later in the day to complete my check-in, get into the room, plop down and relax.  I would think that most upscale hotels would hold your luggage (as long it passed inspection by the drug-sniffing, bomb-sniffing dogs).