I had a couple of situations in a game recently where the draws were straight out of a textbook (and they also went great for me), so I thought I would just post screens, explain what's going on, and talk about why I did what I did.
Okay, so I'm tenpai (ready to win) here if I discard 4-sou. Ii pei kou (two of the same straight in a concealed hand), one dora. The wait is 3-pin, not the greatest, and this hand has a lot of ways that it can change shape. With Chun as the pair, and with two other pairs in the hand, I am particularly mindful of the possibility of drawing a third Chun. This would boost the value of my hand and allow me to change my wait to 1-4 pin or 2-5 pin as I please.
(1-pin, 2-pin, 4-pin, 5-pin and Chun all change the shape of this hand without breaking tenpai.)
As such, I deal 4-sou and choose not to reach. This hand is worth 3900 if I draw it, 2600 if I don't.
Conveniently, the shape of the hand changes immediately. With this, the only thing left to do is discard 2-pin and reach. The value of the hand is the same (aside from the 1 han from reach), but now the wait is on 4-pin and Chun. I'm pretty unlikely to get 4-pin from the other players, but Chun is another story. Basically, I am giving up the one han from ii pei kou for a reach, a slightly better wait, and the possibility of one han from Chun.
This story ends even more conveniently, with the best possible outcome. My next draw is Chun, so I get ippatsu (bonus for first draw after reach) and the score goes up to mangan (8000).
This one I only want to post as a basic tip. This is an incredible first draw; I'm one tile from tenpai. For me, the choice of what to discard is really easy: it's the 8-man, 9-man group, starting with 9m.
My reasoning is this: I have three complete straights here (123p, 456p, 234s) and two parts of the hand that could become straights (45m, 89m). I could discard 8-pin and wait on both... but think ahead. What happens when I draw 7-man to complete the 89m group? I'm left with a single pair wait on either 4m or 5m. That's a lousy wait, definitely not one to reach on.
On the other hand, if I disassemble this group now, and I draw 3-man or 6-man... then I have a hand with a single pair wait on 8m or 8p, depending on what I discarded earlier. I still wouldn't reach on this, but it's a hell of a lot better.
Here is what actually happened: again, the best possible outcome. I have a three-sided wait: 1-pin, 4-pin, and 7-pin all complete my hand. With a hand this early and a wait this good, there's no way you don't reach. In another first-draw win, I got 1-pin and, thanks to a lucky ura-dora, 12000 points. I only got second place this game.
With the ippatsu and ura-dora (and red five) rules, your biggest hands will often be your dumb-luckiest. There are a lot of rule variants within Japanese mahjong that cut them out, in fact. I like them, though, because they add danger and risk without making the game completely luck-based. And it is a gambling game, you know?