Apply far enough in advance and you may be able to save the cost of your ticket; many music festivals have volunteer work available in exchange for free entry. However jobs like stewarding and litter-picking are popular options each year and usually book out months in advance, so it's best to check the relevant festival web site at the start of the year if you're interested.
If you're willing to graft, there might also be the option of catering or bar work if you have previous experience. Remember; you'll normally need to pay a deposit to make sure you turn up for all your shifts. Keep a close eye on sites like Festival Volunteer or Refresh the West and you could be seeing some of the festival from the other side of the bar. But be warned, festival work can be fast paced and not for the faint hearted!
If you're feeling more charitable, organisations like Oxfam and Water Aid UK have limited places available at some of the UK festivals - meaning you can soak up the atmosphere whilst giving something back.