MHN,
On the light twins I flew it was "positive rate, toe brakes, gear up" as far as I recall. The Boeings with which I am familiar have a low pressure applied to the brakes when the gear up line is pressurised. Stomping on the brakes just after lift off will likely damage things - think wheels stopped from spinning at 170kt tyre speed to stop in a millisecond. How much stress on the truck snubbers!!!
B747s have "tilt switches" on the main gears. They're actually "untilt switches" and 2 untilts means ground mode. There are also WOW sensors on the NLG for a few systems.
Most jets have an "over centre" lock on their gear, but generally it is not very robust so never move the aircraft unless the gear system hydraulics are powered or the pins are in. There is often a lever latch to prevent the gear lever being moved accidentally.
Boeing like to have gear hydraulics depressurised in flight hence the "OFF" position for the gear selector. This removes pressure from the gear system and allows the gears on most Boeing jets to hang on the up locks provided for the purpose. It makes sense to remove 3000psi whenever possible. If it's not pressurised, it can't leak.