Well firstly, congrats!
My two are 21 months apart and I love it. At times it is hard but I wouldn't have it any other way. The first few months are hard (particularly as I breastfed and DH worked away a lot) but watching their bond develop has been great. They adore each other.
Some advice:
I bought DD1 a doll and accessories so she could play babies alongside me.
We read a couple of books about getting a new sister.
Get baby stuff out well before LO arrives so toddler can get used to it.
Try to get toddler into a routine that you think will work for you once baby arrives and make sure you don't plan any big changes shortly after baby arrives (dropping nap, potty training, big bed, starting nursery etc)
Prepare a basket in/near the main living area (if you don't already have one) with nappies, wipes, change mag and spare baby clothes so you don't have to keep running upstairs.
Have a box of 'feeding toys', a box with special things in to occupy toddler while you're feeding baby.
Line up family and friends to take toddler off your hands for an hour or so every now and then so they get a special treat and you get quiet time with baby.
Find some clubs that you can take both to so you get out of the house and toddler is entertained.
My biggest piece if advice though? Relax!!! It's not necessarily going to be as difficult as you might imagine and you'll get through it one way or another
.