dd was 18 months when ds was born and i was a nervous wreck at the prospect, even started a thread on it!
it all worked out fine and dd has been brilliant, far better than i thought she'd be.
the best advice i can give would be to toddler proof whatever room you'll spend most of your day in and have a supply of nappies down there with you. that way you can let toddler run loose while you feed without having to jump up and down and you don't have to tackle climbing up and down the stairs with both for every nappy change.
be prepared to have to let the new baby cry sometimes while you deal with ds, especially around bathtimes and stuff where you simply can't get to him. it doesn't damage them for life and most of the time you'll find they fall asleep before you eventually get to them.
talk to ds loads about the new baby, let him cuddle and play with the bump and if you plan to bf explain to him lots that baby will be feeding from you breasts, you might not think he'll understand but you'd be surprised. before ds was even born dd was holding her teddies up to my boobs for a 'feed' so she wasn't upset to see ds feeding and she was so used to te idea of a baby coming that when we arrived home with him she ran straight up to the carry cot, smiled and pointed at him, said 'baby!' and gave him a big kiss. i can honestly say there hasn't been one second of jealousy since he was born and she's very gentle with him.
the only other thing i'd say is to make sure not to yell at ds if he does hurt the new baby, on purpose or by acciadent. it's hard not to get overprotective and shout but it only worsens the situation and upsets everyone, i find simply pulling dd away and saying 'no, ds doesn't like that, he likes this' and showing her how to stroke him gently is far more effective.
congratulations and don't worry, it'll all work out when the time comes