How do I skip an iteration of a `foreach` loop?

You could also flip your if test:

foreach ( int number in numbers )
{
     if ( number >= 0 )
     {
        //process number
     }
 }

You want:

foreach (int number in numbers) //   <--- go back to here --------+
{                               //                                |
    if (number < 0)             //                                |
    {                           //                                |
        continue;   // Skip the remainder of this iteration. -----+
    }

    // do work
}

Here's more about the continue keyword.

---

Update: In response to Brian's follow-up question in the comments:

Could you further clarify what I would do if I had nested for loops, and wanted to skip the iteration of one of the extended ones?

for (int[] numbers in numberarrays) {
  for (int number in numbers) { // What to do if I want to
                                // jump the (numbers/numberarrays)?
  }
}

A continue always applies to the nearest enclosing scope, so you couldn't use it to break out of the outermost loop. If a condition like that arises, you'd need to do something more complicated depending on exactly what you want, like break from the inner loop, then continue on the outer loop. See here for the documentation on the break keyword. The break C# keyword is similar to the Perl last keyword.

Also, consider taking Dustin's suggestion to just filter out values you don't want to process beforehand:

foreach (var basket in baskets.Where(b => b.IsOpen())) {
  foreach (var fruit in basket.Where(f => f.IsTasty())) {
    cuteAnimal.Eat(fruit); // Om nom nom. You don't need to break/continue
                           // since all the fruits that reach this point are
                           // in available baskets and tasty.
  }
}

Another approach is to filter using LINQ before the loop executes:

foreach ( int number in numbers.Where(n => n >= 0) )
{
    // process number
}