The easiest way to do this is to interpolate your data to a higher resolution using an interpolation scheme that will result in smoother transitions between points, such as a spline.
[x,y,z] = peaks(10);
[xnew, ynew] = meshgrid(linspace(-3,3,100));
znew = interp2(x,y,z,xnew,ynew, 'spline');
subplot(2,1,1);
hold on;
scatter(x(:), y(:), [], z(:), 'filled');
contour(x,y,z, -6:7);
subplot(2,1,2);
hold on;
scatter(x(:), y(:), [], z(:), 'filled');
contour(xnew, ynew, znew, -6:7);
Be careful with this, though, since it might introduce some artifacts. Even though it may not be as visually pleasing, the low-res, less-smooth version is a more accurate depiction of the underlying data.
