I disagree with some of the things Red Rooste5 said. I've been there an awful lot (lvl 88 Thieving) and this is how I did it.
First, I suggest that you complete the 'Contact" quest, which allows you to acces a bank right near Pyramid Plunder. I guess you don't have to, you could always use a Ring of Dueling and take a Magic Carpet to get back. The bank makes it way easier to restock on food.
Now, I always wore nothing except a Spottier Cape and Boots of Lightness, mainly because I didn't bother with the Gold Chests or Sarcophagus'. Really, if you want to train Thieving, there's no point in wasting your time for a 1/100000 chance of a Sceptre. The monsters Mummies and Scarabs just get in the way. What I bring is money for traveling, one Waterskin (discard when you get in the city) and a full inventory of Lobsters. The Super Energy's are a waste of money and space, and believe me, after getting there, you want to stay as long as possible, so you need food. The Anti-Poisons are pointless too. Not only do they waste space, but the poison barely takes off any damage and wears off rather quickly. Personally, the Lockpicks never helped me, so do whatever with that.
So now that you know what to bring, here's how you actually take on the Plunder. First you must find the correct door into the Pyramid. After you have found it (one of four doors) that door will remain the correct door for a couple runs (This doesn't apply to the doors inside). After a while, the doors will switch. Right when you get in you are being timed, so get moving. Click on the wall in front of you to disable the trap. If you're reall set on getting that Sceptre, then just open the Gold Chest then find the correct door, fast. The Sarcophagus is way too slow and having a Mummy and Scarabs coming after you while searching for the right door is impossible. Basically repeat this until you get to the last level you can. Open all the urns, even if you get poisoned and bitten, open all of them. Ofcourse, eat when needed (I usually save it for when I get out, as to not waste time inside). Once all the urns are open, proceed to open the Sarcophagus then the Chest if you want. Make sure to open them in that order, you won't be able to open the Sarcophagus with Scarabs attacking you, but you can open the chest with a Mummy attacking. Now you just run to the exit and keep clicking where the message will come up, because if you're too slow, you'll get attacked and the message will dissappear.
My strategy is to get as many runs in as possible in the least amount of time, in order to get lots of EXP, which is why I run from the enemies and usually don't loot the Sarcohpagus. Hopefully I've enlightened you on the process of Pyramid Plunder, you can Pm if you have any other questions.