Timing A little forward planning is key for sparklers. You want it to be dark enough outside to get the right vibe but you don't want it too late as everyones organisation only gets worse with more alcohol.
Bigger the better The main ingredient to any sparkler send off are the sparklers itself. Get short ones that last 15 seconds and you may as well kiss the epic sparkler dream you had away. You want to be looking for sparklers 30cm in length that last around the 60/90 second mark if not beyond.
Light it up Does your venue have a fire pit? Or will you need a some lighters/matches? It seems obvious but believe me when I say often the actual fire can be forgotten.
Let your photographer know Sparkler send offs are known to be a tricky one to photograph however, any experienced photographer will know what they need to do to capture the magic of the moment. Let them know before hand and include it in your timing for the day. You can read a sample of wedding timings here.
To Tunnel or to Not Pro's of a tunnel: Plenty of room for activities Pro's of a crowd: Easier to organise. Personaly I prefer a tunnel. This tunnel is basicaly one the most exciting cat walks you will ever grace.
Finally: ENJOY IT! I won't lie when I say the organisation and prep can be a little tricky (well more so than most images on a wedding day), but book the right photographer and plan ahead it will be one of the most exciting parts of the day for everyone. Once everyone is lit you need to treat that tunnel like a Gucci catwalk! Have fun and go wild!