I use real fruit that I clean and cut into appropriately sized pieces. The process I have developed over several batches of fruit beer is as follows:
1. I will buy the fruit fresh, and then immediately wash it and cut it into smaller pieces so that it is easier to handle for later stages, and also because I want to expose some of the cells of the fruit to the beer. At this time I will also discard any fruit that looks overripe or has exposed surfaces where some kind of infection is likely to have occurred. For stuff like blueberries, blackberries, etc. I will "distress" the fruit instead of cutting it by smacking it a few times against a sanitized surface.
2. I then put the fruit into a sanitized tupperware container in the freezer for a day or two; basically some amount of time on the order of a few days before I plan to rack my beer to secondary.
3. I take the fruit out of the freezer, open up the sanitized container, stick the block o' fruit into a sanitized nylon mesh bag, seal up the top of the bag so the fruit pieces can't get out, and then put that into my secondary fermenter and rack on top of it.
4. Wait some amount of time for the fruit flavors to get into the beer. For some beers this can be something like one or two weeks, and for others it can be more like a month to six weeks. With strawberries, for example, I don't feel like the flavor is imparted very easily into the beer, so I rack it for longer.
5. When I am ready to package, it is then very easy to pull out the fruit bag and clean out the bag and fermenter.