Neutral Density Filters
This method is probably the safest of all and is quite effective. It involves you purchasing one or more neutral density filters and screwing them on the front.
A neutral density filter has the effect of reducing all light, visible and invisible, entering the lens. They are generally sold in different grades such as 2,6 or 8. If possible, buy a 6 and a 2. Then you can add or remove the grade 2 depending on the light. Alternatively, an ND8 may be used with certain filters alone to give a correct light level.
Remember that the net effect of more than one neutral density filter is calculated by multiplying them. So an ND2 and an ND4 is the same as an ND8. Likewise two ND4 filters is the same as an ND16.
The factor number (e.g. 2,4,8 etc.) refers to the light reduction factor. An ND2 cuts light by 50%, an ND4 by 75% etc...
This method seems to be most successful when used with the Hoya RM90 filter as it tends to block out a larger percentage of all light than most other filters, but as with most filter considerations the lighting conditions you film under effect this heavily.
For more information about neutral density filters and their application see the Neutral Density Filters review section.