Unfortunately there are no short cuts to training the Runescape skill of Slayer to 99. However, there is a way for you to generate 100 million GP+ training Slayer, while also gaining opportunities along the way to stop and obtain drops worth millions of GP. So, whether you want to train Slayer to 99, or you just want to get your hands on some rare and valuable drops from Slayer monsters, this is your guide to where the money is at when using the Slayer skill in Runescape.
To train Slayer, you need to get assignments from one of the Slayer Masters located throughout Runecape. Once you get to level 50 slayer and you have 100+ combat, you can stick to the highest slayer master named Duradel. Duradel is in Shilo Village located on Karamja Island, near the southern tip. He gives the hardest and most rewarding assignments.
When you are training from level 1 to 99 Slayer, you are essentially given assignments at random and are forced to complete whatever given task in order to recieve further assignments. There isn't anything you can really do to "rig" your assignments to obtain 
your favorite monsters as tasks, or even to make Slayer training any faster. However, there are things you can do to ensure your chances of getting good drops is the best, making sure you earn as much GP as possible while training slayer to 99.
1.) The Ring of Wealth
When trying to get to 99 Slayer, the Ring of Wealth should be your best friend. The item gives you a better chance of getting rare drops.
2.) Highest Slayer Master
Duradel is the Slayer master to go to for highest-level assignments assignments. If you are not level 100+ combat or 50 slayer and you can't go to Duradel, make sure that you are going to the highest Slayer master you can. This will give you harder monsters to kill. Harder monsters mean more experience for your skills and better drops.
3.) Slayer Items
Although we are not going to explain all of the Slayer items in detail, make sure you use them to your advantage so you can train as fast as possible. Using the Enchanted Gem can give you a better feel for how far you are on a task, so you can begin to get prepared for a new one. At level 50 Magic and 55 Slayer, use 
the Slayer Staff with the Magic Dart attack for a fast, powerful, and cheap way to kill your slayer monsters if you decide or need to use magic. Also, some Slayer items you can purchase are needed to kill some slayer monsters, so keep an eye out for what you need, just so you don't waste a trip by not having the proper items to even be able to kill the monster.
Page two has more information on earning money with the Slayer skill, including the top three moneymaking monsters to kill.