if a headlight or taillight is too tight when sliding on/off the mounting bracket, rub a wax candle on the exposed surfaces. it doesn't take much.

i just did this with a blaze-2W headlight that refused to go on or off the bracket without twisting the bracket on the handlebars... and it was hard to tell when it was locked into the bracket... now it's super-smooth on and off, and gives nice tactile feedback when it locks in place.