The first thing you need to be aware of is sensory stimulation is by far the most important factor in gathering a crowd. If you set up next to something incredibly eye catching like a large screen or fountain, it will be much more difficult for you to capture people's attention. For example, at Vivid festival several years ago I found it impossible to stop people at circular quay or darling harbour as the light projections simply overwhelmed my act. Also, if there is another performance nearby with extremely loud music, this will also cause you problems. You should aim to be as spectacular as possible. Juggling acts that end with the performer riding a huge unicycle have become incredibly common because it is visible from very far away, allowing audiences of a hundred people or more to accumulate. You must also be as loud as possible, ideally with a PA system that plays background music. If do a silent show, people will only be aware of you if they happen to be looking in your direction, however if you are making sounds, everyone within hearing distance will be aware that a performance is taking place.
Many buskers will attempt to attract an audience by talking to random people walking past, inviting them to stop and watch, advertising and talking up their own show. This can be very effective especially if the performer is funny. Personally, I prefer a more passive approach. I tend to just start my act and wait for people to stop and watch. I have several tricks or routines that I use mainly for the purpose of gathering the crowd, like a trick with coins and cards or the cups and balls. I do these on the ground on a mat and do not look up at or talk to people walking by. I feel that this is less intimidating, people are less hesitant to stop and watch because they invest less when they do so. It is easier for them to walk away as the neither of us have acknowledge each others presence. I only do this for the first five minutes or so until a small group has stopped, I then introduce myself and announce that I will start my actual show. I then describe the big finale to give them a reason to stay for the whole show.
Several months ago I met a very talented busker from Hong Kong who was doing a soccer juggling act. His crowd gathering technique was simple but very effective. He would place a bag full of soccer balls four meters or so in front of him and put on some suspenseful music. The guy would just stand there with his hands in front of him like he was about to run and grab one of the balls. It created a great amount of tension, you could tell something big was about to happen. Eventually, over about 2 or 3 minutes a group of 20 or so people would stop and he would just suddenly rush over to the bag and start his incredibly complex juggling show. Sometimes less is more, seeing him perform made me question why I just start doing magic when nobody is watching yet. This has always been effective for me, but maybe I am working too hard.