How to prevent knee/shin guards slipping?
The top strap should go above your calf muscle, and it should be tight enough to not fall down.  Don't depend on stretchy elastic.

Do they reach down to your instep or do they sort of float?  You might get some benefit by tucking the bottom of the guards into your socks.

Another option is tape - http://www.hockeymonkey.com/hockeytape.html  would do the job.

The only knee guards I wear are sports style tube ones, not armour.  However I do wear thin gloves all rides, and have worn elbow pads in a MTB race.