This doesn’t have to be money, you can swap work for exposure, experience (internships), materials, space and favours (I’ll swap you an illustration for a short film of my process). Just be sure that you are always aware of what it is costing you and that those receiving the “favour” know what the value of that work would be, otherwise they will always assume it’s free, to your detriment and to the trade that you belong to. The best way (and you’ll want to kill me for this), is to invoice them for the value of your work, and then cross out the value marking it as “Free this time” (or a reduced price). This will allow you to measure how much of your work you are giving away for free each month, which you should consider as “marketing budget” (all that really means is getting exposure in the first place).
Making people know what you would charge, will remove any shocks when you start charging, make them aware of how much you are really giving them and most importantly give value to your trade as a whole (if they know they can get work for free all the time, why should they pay…enter spiral of doom!) Your allocation for free work for “exposure” should not make up more than 20% of the money you make in that year. (like giving £2 away on each £10 made to help get your work recognised).
Also, be aware that all offers for “exposure” are not the same (showing your work to Charles Saatchi or some guy named John Smith makes a big difference).