Everyone loves shopping for new gear that we all convince ourselves is the path to instant inspiration. And sometimes that's actually true; it's hard to beat the feeling of a new gizmo, especially when it lets you do something you previously couldn't, or even just relieves some annoyance or difficulty you have with your gear. Luckily, there are loads of no-cost DIY or very cheap buys that you can make for a fraction of the cost of a couple of kilos of new glass.
Store small radio triggers in a hard glasses case
Elinchrome Skyports are great little radio triggers for syncing your camera's hotshot with external strobes. They are cheaper and lighter than Pocket Wizards and I've never had a problem with them – except the on/off button on the receiver is really easy to accidentally press, potentially running down your battery in your bag. My solution was to pack a receiver, a trigger, a short PC sync cable and a spare watch battery for the trigger into an old rigid plastic glasses case. It's a little bit fiddly to open and curl the cable correctly to fit it back in, but it's a snug fit. And anyway, if you are setting up strobes with radio triggers, you're not pressed to shave seconds off your shooting time.
Store four AA batteries together in a plastic case
I love these little plastic cases for AA batteries. Sometimes batteries you buy come in a case like these, but if not, you can buy them on Amazon for almost nothing. They make packing batteries much neater and easier. You could also wrap some of your cases in coloured tape to indicate that the batteries have been used. (My case has a little red plastic toggle designed to let you indicate if the batteries have been shot, but it's far to loose to use.) Load your strobe with fresh batteries and bring an empty taped case for the used batteries when you swap in a fresh set.
Make a tiny lens-mounted gobo for triggering strobes from your camera's in-built flash
This is admittedly a gizmo for a somewhat rare edge case, but it costs nothing and takes up no space in your bag. Imagine you're shooting a portrait of someone with glasses, and you've carefully positioned a strobe in an umbrella or softbox to avoid a reflection. If you're triggering the strobe with your camera's in-built flash, even in Nikon's Commander mode in which the in-built flash does not contribute to the exposure, you will still see its reflection in your image. To prevent this, cut two slits in a business card and fold down the bottom 1cm. Thread a rubber band through the slits and position the rubber band around your lens, directly in front of the flash. The signal from the in-built flash happily bounces behind you and doesn't prevent the triggering of the external flash (as long as you're indoors).