After playing, condensation collects in the instrument from the interaction between hot breath and cold brass. You should therefore remove the slides after playing and drain the water. You can store your instrument after it has drained, always with the slide facing upwards. If you take a longer break from playing, you can oil the slides as a preventative measure.
On another note, A tuba should always rest on the bell and be placed to lean against a wall, for example, so that it does not tip over and get damaged.