Beginning your selector with an ID is always best.

// fast
$('#container div.robotarm');

// super-fast
$('#container').find('div.robotarm');

The $.fn.find approach is faster because the first selection is handled without going through the Sizzle selector engine — ID-only selections are handled using document.getElementById(), which is extremely fast because it is native to the browser.

Be specific on the right-hand side of your selector, and less specific on the left.

// unoptimized
$('div.data .gonzalez');

// optimized
$('.data td.gonzalez');

Use tag.class if possible on your right-most selector, and just tag or just .class on the left.

Avoid excessive specificity.

$('.data table.attendees td.gonzalez');

// better: drop the middle if possible
$('.data td.gonzalez');

A "flatter" DOM also helps improve selector performance, as the selector engine has fewer layers to traverse when looking for an element.

Selections that specify or imply that a match could be found anywhere can be very slow.

$('.buttons > *');  // extremely expensive
$('.buttons').children();  // much better

$('.gender :radio');  // implied universal selection
$('.gender *:radio'); // same thing, explicit now
$('.gender input:radio'); // much better