Warping of bottom of print
The first thing to understand is what causes warping.  Warping is caused by the thermal contraction of the plastic when it cools down.

Simplifying things a fair bit, you can visualise the process like this:


hot, expanded plastic gets deposited on cooler, shrunk layers,
when the hot plastic cools down, it shrinks and pulls the upper part of the layer below inwards
at this point, the layer below has a differential in the compression between its upper and lower parts, and curls up
the problem is exacerbated at the very first layer (the one touching the bed) as this is "locked" to a rigid body (the bed) and cannot shrink, while subsequent layers are only attached to the somewhat flexible plastic beneath, and thus can contract.




Also notice that the larger the part being printed, the stronger is the force trying to curl-up your print.

Once one understands all of this, then it is possible to appreciate the many ways the problem can be mitigated.

Here are the common ones:

USING A MATERIAL WITH LOW SHRINKAGE COEFFICIENT

This translates in smaller tensions and thus less force "pulling up" the corners of your print.  Historically, 3D printing started with ABS because this material was one of the very few, relatively safe ones to source.  Nowadays there are materials like PETG which have similar mechanical properties to ABS but are much easier and forgiving to print with, so - unless you need ABS for some very specific reason (e.g.: acetone smoothing) consider never printing with it.

DECREASING THE THERMAL DIFFERENCE BETWEEN MOLTEN AND SOLID STATE

Concretely, this means lowering the "gap" between the ~200°C of the nozzle and the ~20°C of room temperature by using a heated bed and - possibly - an enclosure.

The heated bed not only drastically diminish the shrinkage of the first layer, but because heat radiates, and hot air goes upwards, the entire bottom of the print has shrinkage mitigated.

An enclosure just increase the benefit of the heating bed, by reflecting IR radiation back towards the print and preventing hot air to escape.  A heated enclosure just improve things even further.

Some slicers offer a "shroud" option, that encloses the entire print in an enclosed, sacrificial structure, that tries to emulate the benefits of a proper printer enclosure.

INCREASING ADHESION WITH THE PRINTING BED

That is the "brutal force" approach: if you face a strong "curl up" force, oppose it with a strong "anchor down" one.

The increase in adhesion can be achieved in a number of ways:


Lower print speed (more time for the molten plastic to "bond")
Overextrusion (more pressure, more material)
Disabling cooling fan (more progressive cooling, more time to "bond")
Using a brim (more contact surface between print and bed)
Using "ad hoc" material on the bed (PVA glue for PLA, ABS sludge for ABS, kapton tape, hair spray, blue tape, etc...)


REDUCING THE CURL-UP FORCE

This is typically achieved during design.  Designing is a vast field and it would be impossible to cover all the possible mitigating strategies one could use, but here are some of the most common ones:


Prefer assembling smaller parts over printing huge ones.  This is self explanatory really, as the curling force increases with the amount of material "pulling", the least material one has, the less force one gets.
Make relief holes above the first layers in long structures.  This will essentially "break" the build-up of tension in the layer, creating many points with a little "curling up force" rather than two with a huge one.  Something along the lines of this, for example:





Avoid extensive overhangs close to the bottom of the print (this is because otherwise you will have considerably more material "pulling up" than you will have "anchoring down".  Here is an example of what not to do (to be fair: this was specifically taken from a bed adhesion/warping test).




Of course all of the above strategies can/should be combined, when possible.  Even if not warped, a part with a lot of internal tension will perform less predictably and possibly worse than a part where such tensions are lower.