Depending on your magazines, you can tape them together. What I do (and also the majority of clip-using players) is tape two 18-round magazines together, with one upside-down from the other. This allows you to have two clips at once, and you just flip the clip 180 degrees to reload. I've also done quad-clips by taping two flip-clips (also known as jungle clips) side-by-side so that there is a gap between the clips facing the same way. Another great advantage is that you can scavenge darts and load them into one of the clips if it's not full, and flip it to a full clip whenever you feel like it. Keeping your magazines topped off is VERY useful, especially when there aren't many safe zones to take advantage of.
If you're using drums, my only advice is to not use them anymore. Sure, you can hold more ammo in a single drum, but they take up way too much space, they aren't very durable if they hit the ground, and most don't feed fast enough for many blasters. Raider drums are the slowest and aren't even that great when slam-fired in Raiders, while the 25-round Rampage drums are fast enough for slam-fire but not fast enough for semi-auto. Raider drums can be taped together as a dual drum, but the Rampage drum is centered and can't be combined with anything else (I've tried). The only drums worth using are the old Alpha Trooper drums, and you can tape them together to make a dual-drum mag. Still, nothing beats the capacity, reliability, and space efficiency of a few jungle clips.
If you want to switch out a completely empty mag under pressure, I would have a dump pouch or some sort of bag that you can throw the empty clip into. Duct tape can be made into anything, and I've made a leg pouch that can hold two jungle clips securely, but it's still easy to quickly drop in the clip and pull out the other. Also consider industrial velcro. One guy at my school uses a lot of it to secure clips to the side of his Longshot, and I bet it would work well on a vest or something. Home Depot has really cheap (yet strong) velcro that's applied like tape and comes in rolls of 15 feet or more. Throw a strip of velcro on each side of your jungle clip, cover a vest or backpack in the opposite side of velcro, and you instantly have a secure yet accessible way of storing full and empty clips.
I don't have any photos, but take a look at this video on jungle clips: http://www.youtube.com/watch?v=jeHv5aBfjfg
Also, I often carry two old Alpha Trooper drums in my back pockets as spares in case everything else is used up. The drum will hang out the top of my pocket, but it doesn't shake loose or get uncomfortable, plus I have the advantage of 36 extra rounds.