This is much easier done in an electric wheelchair, a bit safer too.
We motorized gimps have a much lower center of gravity to contend with. If your electric wheelchair has the ability to tilt and recline then you should use these to your advantage by shifting all the weight you can towards the heaviest end of your chair. Now, when approaching the escalator you want the heavy end to be the high end, so...
going up = heavy end first
going down = heavy end last
Techniques for riding down on an escalator are to varied by all the different types of powered chairs and their ground clearance. Try going up first and let your level of success and comfort determine if your ready to go down. If the chassis of your chair touches the steps when going up, don't go down. You're just not cut out for it. You'll slip off the steps and break your... whatever you didn't already break.
Scared? Don't worry yet; going up is almost kinda mostly easy. Just cram your leading wheels up against the step as it forms. When you're wedged up against the step good and tight release the controls and let your power brakes engage. Be ready to move fast when you reach the top. The escalator is not going to give you a smooth 4 point landing, it is going to fuck with you! You need to be sure your leading wheels are rolling when they reach the floor to match the speed of the escalator.
If you're not synchronized you will get jostled.
If you get jostled you will get crooked.
If you get crooked you will get stuck.
If you get stuck you will get sued.
If you get sued you will deserve it.
(Please don't make the rest of us look bad by whining about discrimination and your right to access. You did a stupid thing and the poor folks that had to pry your dumb crippled ass out of the escalator didn't get paid enough. If a store doesn't have an elevator they must not want your money. Move on.)
Now go enjoy your new found ability. But be careful, you're fucked up enough already.