Be careful what you put in your shocks. ONLY use oil with anti-foaming agents. A couple options are 20-weight motorcycle fork oil which is perfectly suited, or you can buy Armstrong shock oil from Moss Motors, which is what I use.
Once you fill the shocks you shouldn't need to do it again for many years. If it leaks out at all, you will either have to continue topping them up or send them in for rebuild. I also recommend Peter Caldwell of NOS Import for replacements / rebuilds.
If the shocks are very low or completely dry, you will have air trapped inside and the shock will not perform optimally. The best thing to do is disconnect or remove the shock and work the arm up and down until the bubbles in the lower chamber have all escaped to the top.