Any tips on using HE shells?
It's a lot like using HEAT, you need to avoid spaced armor and tracks if you want to do decent damage. But, my best results were always gotten by staying out of normal engagement range and assisting the front line by derp sniping. Unless you shoot gold your DPM and will be pretty low against things you can't one-shot, so hiding is good. Plus, HE ignores range for damage and pen, so once you get a good feel for your flight time you can be almost as effective at 450m as at 250m.
If you do find yourself close to the action, do your best to hit (in order) the top of their engine or top of turret, the back of their engine, the back of their turret, the side of their turret, or their driver's hatch. That's not just where the armor is normally thinnest, it's also where you're most likely to start fires and kill crew (which HE has a bonus to, iirc). As a last resort, you can also try aiming a round right down their barrel, it will often damage or break their gun.
Also, memorize which tanks you can penetrate, and where. For example, T29's large tracks make a good side shot into the hull difficult, but a derp to their rear hull will almost always penetrate and/or start a fire.