The first thing you have to do is get over the fear that naturally comes with playing higher and lower. As you reach the extremes of you range, you will find that you're not really sure if everything will come out as it should. Do your level best to decide that everything IS going to come out, and it will come out the way you like.
With the right mindset, scales are the best way to go. Learning an extra octave or cycling into an extra fifth or third on major scales increasing your range in a way that is "normal" and "comfortable".
Moving on, you're best served by adding regular lip slur exercises to your regimen. Even slurring in a normal range will provide you with greater flexibility to move higher and lower on the instrument. It has less to do with setting your lips for particular notes, and it has more to do with providing yourself the fundamentals to play the WHOLE instrument well.
A good plan is to add one new not to your range every one to two weeks (depending on how often you are able to practice.) Before you know it, notes will be coming out. The higher and higher you go, you'll find that notes that once seemed high now feel "normal".
That is when you've got something.
Frankly, playing consistently in the upper register works wonders. Scales in multiple octaves are great.
Make sure to use faster air, rather than attempting to change your aumbesure (sp?) to reach the higher notes. Use lots of support and work your way up the chromatic scale.
For lower notes, especially pedal tones, relax your lips as much as possible and try to hit below a Bb in first position, there is a note you can hit there without a trigger tbone. Relax the center of your lips while maintaining the corners to prevent air leakage. I personally think adjusting your jaw also helps for low notes but don't quote me on that.
Frankly, practice makes perfect. XD. I've recently had to work on my range as well, and just playing more often in those octaves works wonders.