Seeing how your background is in concrete, and concrete is relate to mortar but not entirely the same since mortars have a much higher surface area to volume ratio, I might be able to help a bit.

What's been said are simple answers. Real answers depend upon what type of mortar you are using, cement plaster (base or skim coat)? tile adhesive? tile grout or joint/crack fillers? general repair mortar? self leveling compounds? masonry (brick-laying) mortar?

Also what kind of strength are we talking about? compressive? tensile adhesion? flexural?

Now for the longer version of the answer:

For improving cementitious bonding:

I. High water loss mortars (thin/skim coat plasters, ceramic tile adhesives for cement based substrate, masonry mortars for brick laying, anything with high absorptive surfaces or high surface area to volume ratio): In this case, it's all about water retention and speed of strength development. Obviously you can increase cement content, use pozzolans, cover up, etc. But single biggest thing is to have high amount of water retention aid (or high viscosity water retention aid) like methyl cellulose derivatives, between 0.3% to 0.6% of dry powder weight. AND you need a lot of water so it can withstand the water loss. But take care to balance water retention aid concentration and water demand as TOO much water will also cause weakness. I can give you more details on that.

II. Low water loss mortars (thick/base coat plasters, porcelain tile adhesives, repair mortars, etc): you will still need decent amount of water retention agent, like methyl cellulose derivatives, but now particle size distribution, water demand, cement content, are just as important.

For increasing adhesion power regardless of cement:

You can add liquid or powder latex (VAE, acrylic, etc). This will definitely improve your adhesion strength, but will negatively affect compression strength somewhat. But it will allow for adhesion between organic and inorganic surfaces, as well as non-absorptive surfaces, such as tile-on-tile adhesion or porcelain tile on glass adhesion. Adding latex binders will also help with flexural strength, such that vibrations and expansion/contraction will not breakdown as fast.

Well, obviously, using some pozzolans such as Silica fume or Fly ash are effective in producing more Calcium-Silicate-Hydrate (C-S-H) gel and consequently increasing compressive strength of the obtained mortar. Note that most of the strength comes from C-S-H. According to my own experience, as well as earlier studies, Silica fume is more effective than other conventional pozzolans. So at the first step, replace 10% of your cement with Silica fume (replacement of about 10% to 20% was recommended).

Moreover, reducing W/C ratio by using superplasticizer is also quite effective, even more than Silica fume.

Note that Silica fume is an expensive material so reducing W/C ratio is more effective both in terms of technical and economical points of view.

In addition, adopting a good curing technique has some effects. For example if you are referring to precast mortars, autoclave technique would have a good effect on strength of the produced mortar.

actually, with decreasing rate of W/C and have enough curing and suite implementation and adding pozzolans suitable with your purpose you can get the best result

Use of nano materials upto certain amount.. read available articles for details...

Proper curing and slow drying of the mortar. if you expose it to the sun the rate of drying will be faster then the mortar will become weak

Seeing how your background is in concrete, and concrete is relate to mortar but not entirely the same since mortars have a much higher surface area to volume ratio, I might be able to help a bit.

What's been said are simple answers. Real answers depend upon what type of mortar you are using, cement plaster (base or skim coat)? tile adhesive? tile grout or joint/crack fillers? general repair mortar? self leveling compounds? masonry (brick-laying) mortar?

Also what kind of strength are we talking about? compressive? tensile adhesion? flexural?

Now for the longer version of the answer:

For improving cementitious bonding:

I. High water loss mortars (thin/skim coat plasters, ceramic tile adhesives for cement based substrate, masonry mortars for brick laying, anything with high absorptive surfaces or high surface area to volume ratio): In this case, it's all about water retention and speed of strength development. Obviously you can increase cement content, use pozzolans, cover up, etc. But single biggest thing is to have high amount of water retention aid (or high viscosity water retention aid) like methyl cellulose derivatives, between 0.3% to 0.6% of dry powder weight. AND you need a lot of water so it can withstand the water loss. But take care to balance water retention aid concentration and water demand as TOO much water will also cause weakness. I can give you more details on that.

II. Low water loss mortars (thick/base coat plasters, porcelain tile adhesives, repair mortars, etc): you will still need decent amount of water retention agent, like methyl cellulose derivatives, but now particle size distribution, water demand, cement content, are just as important.

For increasing adhesion power regardless of cement:

You can add liquid or powder latex (VAE, acrylic, etc). This will definitely improve your adhesion strength, but will negatively affect compression strength somewhat. But it will allow for adhesion between organic and inorganic surfaces, as well as non-absorptive surfaces, such as tile-on-tile adhesion or porcelain tile on glass adhesion. Adding latex binders will also help with flexural strength, such that vibrations and expansion/contraction will not breakdown as fast.