Following are a couple of tips to help your overtone practice, though the first will also help with your sound generally.
Low Bb and the Lower LipFirst, I'll start with overtones on low Bb, which can be sometimes be squirrelly or unstable. Essentially, overtones on low Bb can be more sensitive to embouchure pressure than overtones on other fingerings. That means if your embouchure is putting too much pressure on the reed in some form or other, the first octave overtone may be completely elusive. Similarly, the second octave overtone can also be somewhat difficult, especially for beginners.
One way to combat too much embouchure pressure is simply check your lower lip position. Often, with more lower lip rolled in against the reed, it is easier to exert pressure on the reed and there is more contact between your lower lip and the sensitive part of the reed, since the lip is necessarily closer to the tip of the reed. Try rolling your bottom lip further out then normal. For example, when you say a long "v" sound you place your top teeth on your bottom lip in the same location that would be good for saxophone playing. Try recreating that same bottom lip placement on the reed. Another method is to form your bottom lip like you're sucking your thumb. That also creates a good bottom lip placement.
Overtones on D
Various overtones on D are out of tune and difficult starting with the second octave and higher. Recently, I reviewed a manuscript by Mark Lanus on altissimo, and he suggested using the low C# key along with the D fingering to stabilize the D overtones. I've tried it on tenor, and it works really well. Thanks Mark (and thanks to Bret Pimentel for his fingering diagram builder)!