need help building dirt jumps

wd40 may work for a whack or two but it will wear off.

stacking up the wet, sticky dirt shouldn't be a huge problem and is your friend, you'll just have to get dirty to get the pile in place. if the dirt is sticky when you're trying to shape the lip or landing, use a light dusting of some clean, dry dirt on top of the wet...kind of like using flour when baking. the dusting of dry will let you pack and shape w/o anything sticking to the shovel. too little dry/dust and it will stick. too much and you may dry things out or create weak spots. you'll just have to practice and figure it out.

WD40 might work, but it doesn't sound very nice for the local flora and fauna. Often, premixing your dirt and water into not mud, but something JUST before mud, helps for the stacking. Also, in delicate situations, stacking your non mud by hand instead of by shovel can help with efficiency. I've never actually done it, but I've always wanted to shape a lip with zero waste by stacking sand bags and covering and shaping just the riding surface. Mixing in a bit of Portland cement can help in difficult situations as well.

Oh... And bigger and higher are almost always recommended. So you're on the right track there. My favorite day of digging was just me and my friend Garren, out at the old De La Jumps in Santa Cruz. We were out in the worst winter weather just throwing mud for hours when we finally hit good dirt. Then we feverishly dug until each lip and landing of the first two jumps were two meters tall. (Measured against my height.) All near vert. Good riding, well earned.

Do work. Do good work. Do good work well. Then ride and enjoy.

I typically stack dirt with a spade, then switch to a flat head shovel for shaping and packing. When it's time to pack, the dry dirt technique can work well as Spomer mentioned, but also try smacking and smearing the dirt at the same time. You kind of just push forward slightly as the back of the flat head hits the dirt. This helps prevent the dirt from sticking to your shovel. With time you'll get pretty good at it. Pack until your shovel starts pinging!

Ping! Ping! I love that sound.