How to control bacterial contamination?

In what? Food is treated radically different from eg. surgical
instruments. Please specify a bit more.

Here's a simple answer to your open-ended question:

Bacterial contamination is typically controlled by using an autoclave
(uses wet head), irradiation (bombardment with subatomic particles
damages molecules), uv light (causes dna damage), antibiotics (most
were discovered from molds), chemicals (e.g. ethanol, chlorine),
refrigeration (at <= 5 Celcius growth is practically halted), or by
controlling nutrient/gas requirements (e.g. lack of oxygen would kill
obligate aerobes).

Which method depends on the application/purpose.  For example, are you
trying to sterilize your growth media?  Are you trying to selectively
favor the growth of one organism and supress the growth of others? 
Are you trying to preserve food from spoiling?  Are you trying to
avoid infection during an operation?

Aseptic technique is a set of practices used by biologists to limit
contamination, both of the biologist him/herself and the sample(s)
being handled.  It involves, for example, wiping down surfaces with
70% ethanol (10% bleach works too), working under a laminar flow hood,
avoiding any contact with skin (e.g. wear gloves), etc.

Good luck,
Ed the biologist

Everything that Ed stated above is correct but I would also like to
add that you can use what's known as a flow hood to keep out bacterial
spores that may contaminate your experiment. Flow hoods pump steriled
air over the experiment table keeping any airborne particulates away
from your testing substance. Away from the hood you have to make sure
that the experiment container is sealed. Combining this technique with
the ones Ed listed above will normally do the trick unless you are
using particularly susceptible growth medias.
Cheers,
Todd the Geneticist