When you can’t bounce your on-camera flash off of a wall or some white surface, I strongly recommend you use some type of diffuser on your flash unit. Remember, we are talking about external flash units which you have attached to your camera in the camera’s hot shoe, not your built-in flash. Although, if you must use the built-in flash, by all means drape a tissue or napkin over it to diffuse the light from it some. No, we are talking about a larger flash to which you attach some type of diffusion material or equipment.
The first diffuser most photographer’s get when they start using flash is something like the StoFen, a translucent plastic piece that fits over your flash head and spreads the light from your flash 360 degrees. These work and certainly are an improvement to a bare on-camera flash, but I prefer diffusers that act much like bouncing your flash off a wall. There are several of this type, but my favorite is the ExpoImaging Rogue FlashBender (http://www.infotor.com/blog/flash-friday-rogue-flashbender-what-would-i-do-without-you/).
The FlashBender is my go-to diffuser whenever I shoot events. Attached to my upright on-camera flash sitting in the hot shoe, it does look a bit unwieldy but it gives me consistent and great results. In these situations, I am shooting in E-TTL mode on the flash and usually Manual mode on my camera. In this way, I can play with the shutter speed to control the ambient light, making it darker or lighter as the scene dictates. I choose my aperture depending on the depth of field I need for a shot. Quite often in a normally lit room - in a house or hotel reception hall - I will shoot at ISO 800 and my Canon 60D gives me shots with great resolution.
There are also many DIY ("do it yourself") alternatives to these diffusers. Though some are a bit cheesy looking and you may only want to use them in a pinch, a few can look as professional to the casual observer as their commercial counterparts. The "using in a pinch" situations might be when you are at a gathering or event where you are called on to take some shots but you don't have all of your gear with you. On such an occasion, you will be forgiven - nay, when admired, perhaps - if you throw a white coffee filter over your flash to diffuse the light or grab an empty plastic milk container and use a knife to fashion a quick DIY diffuser! The best and in my opinion most professional knock-off is of my favorite FlashBender. This can be made easily by simply cutting a piece of white foam sheet (available at craft stores such as Michael’s or AC Moore in the U.S.) and fastening it to your upright flash with a rubber band, hair scrunchy, or tape. Alternatively, you can use a piece of 8.5X11" 60 lb. paper with a wrap-around cut to go around your flash head that can be secure, again, with a hair scrunchy or rubber band (see accompanying photos).
In addition to teaching scheduled photography classes throughout the year, I am also available to do one-on-one tutoring or small group lessons designed to meet YOUR needs and what you want to learn in the area of photography, using flashes, or the use of Apple products and software. Give yourself the gift of learning: http://www.infotor.com/photoclasses! Check out my new special tutoring bundle: http://www.infotor.com/photoclasses/tutoringbundle.php!