Working on your antenna will naturally involve working at height, probably with the aid of ladders. Ensure before you commence work that you are familiar with the safe use of ladders, and always do this work accompanied by an adult. Working up a ladder alone is foolhardy, a person at the bottom of the ladder will be able to steady it, monitor it for movement, and if the worst happens, summon help.
Keep the antenna feeder and the antenna itself clear of being walked into or touched. If anyone can touch the antenna elements, or their connection to the feeder they may sustain an RF burn. This applies equally when the elements are insulated or made of bare wire. RF burns are almost painless at the time, but quickly afterwards become very painful and take an extraordinarily long time to heal.
Stay clear of all overhead lines. When erecting an antenna, consider what may happen if it becomes detached or falls down. If it were to touch a power line, then you may, if operating at the time, receive a fatal electric shock, or have a fire.
Wind loading on antennas may be much higher than you realise. During a gale or strong winds, very large forces are imposed on the antenna and its support structures. Will yours stand up to the battering. The time to think about this is during the planning phase. Ensure you have sufficient guy ropes on large masts and that they are up to the job - make sure these guy ropes do not themselves pose risk of injury.
High antennas and masts also risk lightening strikes. A lightening strike may cause a massive current to flow through your feeder and radio to earth. Contact your local planning department for advice. as well as local radio clubs and dealers. Regardless of what protection in the way of lightening arrestors etc. is fitted, it is advisable to disconnect antennas when not in use.