I would assume that buzzing the tower and general hot dogging around the airport is prohibited, so how do airshow pilots get practice? Do they practice at high altitudes? If that is the case, how do they practice stunts that by nature require low altitude, like Bob Hoover's waltzing landing?
Aerobatic pilots typically use "aerobatic practice areas" when they practice. Many of the Part 91 rules which would need to be broken in order to practice aerobatics are waived within these aerobatic practice areas (such as the 1,500 ft. "floor" for normal aerobatics in 91.303(e)).
The FAA has to approve them, and their guidance includes a good description:
4. AEROBATIC PRACTICE AREAS. Aerobatic competition pilots, airshow pilots, and others who wish to practice aerobatic maneuvers not necessary for normal flight and below an altitude of 1,500 feet above ground level (AGL) must use a waivered aerobatic practice area. These areas are not to be considered airshow sites. The aviation community uses these practice areas to establish and maintain proficiency as well as enhance competitive skills in all the recognized aerobatic maneuvers. They are established by the waiver applicant in conjunction with the local FSDO and may have dimensions of several miles in various directions or be as small as a contest box; i.e., a cubic box with a dimension of 3,300 feet on all sides. Inspectors should be receptive to the establishment of these areas, consistent with safety and the efficient use of the NAS. It is imperative that the safety of all nonparticipating aircraft be considered when issuing a Certificate of Waiver or Authorization for an aerobatic practice area.
An aerobatic practice area may be established over an airport if coordinated with airport management, and can be used to practice landings like those that Bob Hoover does.
Besides the special rules in these areas, it is also a good way to warn other traffic that aerobatics could be taking place because they are listed in the A/FD and a NOTAM is issued whenever they are hot.
A lot of the maneuvers can be learned and practiced at higher altitudes.
In the US, the airshow pilots usually request a waiver from the FAA (to waive certain rules) so they can practice at lower altitudes. The waiver will specify the location, hours of operation, who can use the waiver, which specific rules are waived (usually aerobatics within 4 NM of an airway, and/or certain altitude restrictions).
In the US, a lot of airshow pilots get their start competing in IAC aerobatic contests. This gives them the opportunity to learn the maneuvers and get some very experienced critiquing. As they progress in the sport they attempt more difficult maneuvers at progressively lower (but still safe!) altitudes.
In addition to what's already been mentioned, there's usually at least one day and more often several days of practice sessions at the field where the show is to take place before the actual event.
That allows the crews to familiarise themselves with the actual local conditions as well as the visual cues ("hey, there's a tall red chimney right where I need to start my turn, handy").
And for "buzzing the tower", you can of course practice that with anything really. A nice tall free standing tree for example, a chimney of an abandoned factory, anything tall and slender in an area where you're allowed to practice.
Or lacking that mark a spot on the ground and in your mind extend that up.
And oh, you can practice low altitude things at high altitude the same way, create an imaginary plane at say 10.000 feet, calibrate your altimeter to show 0 at that altitude, and now just fly like you would normally. Safe too, if you get things wrong and would end up flying into the ground, you now still have 10.000 feet before you splatter yourself.