Use of Grenades

[Dr Sadako]

Flashes
Avoid throwing 2 flashbangs (FBs) in a row. Most players are prepared for that.
If you do throw both of them, throw one short and one long. The opponent being prepared for two FBs will turn instinctively at the second and will most probably be blinded.

A combination of HE + FB is usually very effective.  Throw the HE first and then the FB.  The FB is usually unexpected as the enemy will have a tendency to believe that the second grenade is an HE too and therefore may be watching it as the FB goes off!
 
Fight flash with flash! If you know that the enemy is nearby and you notice their first FB coming in, then get ready with a FB yourself (if you have backup). The second flash will most certainly come soon and will be followed by a rush. The defensive theory suggests that you throw your own FB to blind their approach, preferably before their own second FB, and then fall back to a good position.  A more aggressive approach is to follow up your own FB and pre-empt their attack with your own attack whilst they are still blind from your FB.  If you time it right then you can cut a whole squad down whilst they are stumbling around!

Smoke
The most important thing regarding smoke is that you should never throw it at your own feet.   Being in smoke reduces your options as you can’t see what is going on around you and the enemy will see you in the clearing smoke before you can see them!    Throw smoke into a bottleneck of some kind and then pick the less usual camping spot to guard it.   If the enemy goes through the smoke he will first aim at the regular camping spot and this will help you pick them off.  But remember, vary your position if this is a tactic you use repeatedly.

Smoke can be a great offensive weapon and used to pin down the enemy for a while.  For example, on de_inferno Ts could throw 2-3 smokes towards the left site (CTs right) and then keep rushing up the middle. The guarding CTs on the smoked side usually stay there 10-30 seconds and that should be enough to take over the other bomb site and entrench yourself in good defensive positions to guard the planted bomb.

Grenades in Defence
The defensive use of FBs and smoke on some maps is key to stopping an offensive rush. Playing CT on de_dust2 is a perfect example. Bomb site B and the mincer is a regular rush zone right after spawn (as well as short A). Use flash, smoke, and HE to slow down the rush and buying time to get some backup.   Throwing a FB intot he mincer as the Ts rush through is a great way to both stop them and disorient them.   And there’s nothing to stop you killing a few whilst they are blinded either!
 
HE - at the end of a round
Think about saving your HE to the end of the round if possible.  Late in the round the enemy has usually already been involved in several skirmishes and has lower health as a result.  This makes the HE more effective and quite often lethal whereas the same HE at the start of a round would not have killed them.   A bonus is that an HE is usually not expected at the end of the round!