The horn may seem a robust instrument but is surprisingly fragile. It is essential to look after it carefully from the beginning. Following these tips should ensure it remains in good working order.
This should be washed regularly to prevent it getting clogged up with dust and whatever may have been transferred from your mouth (it is perhaps best to avoid eating, especially sugary food, directly before playing!). A mouthpiece brush is inexpensive and a fairly essential piece of equipment.
If anything goes wrong with your horn, it will probably be with the valves. Some manufacturers recommend using three types of oil (rotor, bearing and piston) but this is probably unnecessary. A good valve oil such as Blue Juice or Al Cass Fast is just as good. The important thing is to oil the valves regularly, not just when they begin to seize up. Once every one or two weeks is ideal. To oil the valves, unscrew the valve cap and apply the oil to the screw (the one that moves when the valve is depressed) then turn the horn over and apply the oil to the underside of the valve, ensuring you reach every moving part of the mechanism. For particularly sticky valves you can remove the slides of the appropriate valve and drip oil down into them, moving the valve up and down as you do so.
It is important to keep the slides clean and lubricated so they can be moved precisely for tuning and removed quickly and quietly to empty water (this is especially important in a concert situation). The simplest way to do this is regular application of a lubricant. The best of the specialist lubricants available is the Selmer Tuning Slide Grease (this is VERY sticky - try not to get it on anything but your horn!) but any other slide grease or even vaseline will do.
Every 6-8 weeks the horn should be given a bath in lukewarm soapy water. 'Pull-throughs' can be bought in music shops (paxman.co.uk has all the horn-related accessories anyone could wish for) to clean within the tubes, but a high-powered jet from a shower hose will also do the trick. Once the soap is washed through give it a rinse and reapply the oil and slide grease once dry.