Arranging your sidebar
Having widgets is one thing; arranging them well is what makes a sidebar useful instead of noisy. The arranging happens in Appearance, then Widgets, and the sidebar's general behavior in Configuration, then Settings under Layout & Widgets.
Order and placement
Widgets render in the order you arrange them, top to bottom. The order is worth real thought: the top slot is prime space members see on every visit, and the bottom of a long sidebar is where widgets go to be ignored. Lead with the widget that answers "what's happening right now", because that's why members open the board.
Sidebar behavior
Side: left or right of the board. Right is the classic; left gives the sidebar more attention.
Sticky: a sticky sidebar rides along as members scroll long topic lists. Great with a short sidebar, awkward with a very tall one.
Dismissal: whether members may hide individual widgets for themselves.
Audit your sidebar the way a new visitor sees it: open your board in a private browser window. If the sidebar is taller than the board index next to it, it's carrying widgets nobody's using; retire the bottom two and nobody will miss them.