Assuming that brass instruments were initially well recorded, equalization at the mixing stage should be minimal. How a horn section fits into a mix will determine equalization (EQ) approach. A full-range sound can be warmed by emphasizing the 200-to-400 hertz (Hz) range, and punchy R&B sounds can be enhanced adjustments in upper frequencies, typically in the 1-to-4 kilohertz (kHz) and 6-to-8 kHz ranges. For most natural sound, use small EQ adjustments.
The dynamic range of brass instruments is distinctive and wide. For the most natural sound, it is best to record the instruments without compression or limiting, with lots of headroom. At the mixing stage, volume automation and compression can even out the wildest dynamics. Use short attack and release times, but be careful not to interfere with the natural attack of the instruments. Brass cuts through a mix easily, so a little compression will go a long way.
The flavor of brass is in itself special, so rarely are effects other than reverb added. Reverb helps blend the section, so it is typical to send the signals of several instruments through a common reverb patch. The ambient effect will be a small-to-medium room in most cases, with a pre-delay set to leave attack transients clear. As with all things brass, a light touch with reverb retains the most natural sound in most instances.
The nature of your recording will suggest how brass instruments are staged, or panned, across a stereo spectrum. A brass quintet will be spread from left to right across the stereo field, while a punchy, Motown-style part may have the horns all panned to a specific spot. Fitting brass instruments into empty spaces across your sound stage can be an effective way to create a feeling of breadth and depth. Don't be afraid to revisit EQ, compression and reverb to tweak the brass for your ideal result.