This short guide's primary target group is intermediate level players that play Sedna occasionally, or newbies that wish to learn Sed basics in a correct way from the start. I'm trying to make it reader-friendly, so I'll keep math/strategical planning babble out of the text as much as possible.
The rationale behind writing this guide is most casual Sed players (even very good mids/lower-tier pro players) use Sedna's skills sub-optimally. The pattern I usually see when fighting a Sedna is: spamming Pounce on the enemy > spamming Heal on herself. That is wrong. If you have a crapload of mana, it can be an option, but having a crapload of mana for Sed = feeding any decent opposing team due to low hp. I believe a Sed should either run with 1 helm only (in 3v3 and most 2v2 matchups) or, possibly, with no helms at all (in certain 2v2 matchups and 1v1). Thus, Sedna is not about skill-spamming, but rather about mana conservation and precise timing. Luckily, the nature of her viable active skills is such they all are extremely situational: (I) Pounce is either a finishing move or an interrupt that should be well-timed; (II) Silence is either a sigil blocker, an escape/save mechanism, or an interrupt that should be extremely well-timed; (III) Heal is either an emergency first-aid measure for a dying teammate (in 99% of the cases, not for yourself!) or a debuff remover that should be... yeah, you get my drift.
Let's take a closer look at some practical applications of those skills:
(1) Both Pounce and Silence used as lock/skill interrupts:
http://www.youtube.com/watch?v=ECirZ6QmQH4
Commentary: Nnnils is getting skill blocked by Silence from hitting ppapanek. Portal re-lock and fireball interrupted afterwards. Tries to run away with little hp left, gets killed by ppap's Fireball.
http://www.youtube.com/watch?v=XF7nC7xKdjY
Commentary: Nnnils is attempting to lock the flag. I Silence first, then keep him on target, and as soon as locking animation shows, I Pounce. But nnnils was faking a lock! Oh my gawd! He makes another locking attempt, which gets interrupted by Silence. Two interrupts make a competent Sedna a great lock-blocker (except vs a Shield III-spamming Oak, but that goes without saying).
(2) Pounce as a finishing move/kill-stealer:
http://www.youtube.com/watch?v=_i6ljwCIjWA
Commentary: Enemy Occ ignores Sedna while trying to kill Green-One's Occ. Silence > Pounce (to prevent Blast Off! out of range, as he has a huge mana reserve) makes a quick work of him. In most cases, max Pounce > Silence is a better combo, which can guarantee 1200-1400 damage, depending on Silence level, but for Occ and CoN favor users it's better to use skills in a reverse order.
http://www.youtube.com/watch?v=EZEarquOvcw
Commentary: Here I unabashedly steal a kill from Mandingo 
Again, the principle is locking on a target and monitoring its hp non-stop. When it's < 1000 => Pounce. In fact, stealing kills is 100% justified in skilled matches. Whoever can finish the enemy off, should do it. Don't be a gentleman in combat, better buy your UB his J-Treads if you steal a lot of kills from him.
(3) Heal as an emergency first-aid for a teammate, Silence as a save mechanism (skill-blocker):
http://www.youtube.com/watch?v=q8ZPPm_u5Zk
Commentary: The Heal part is self-explanatory. The Silence part is also quite transparent.
(4) Silence as a 'selfish' escape mechanism:
http://www.youtube.com/watch?v=0EeLUx7oK-c
Commentary: Poor man's Shield III, aka Silence I, is used as an escape option here. Theoretically, it should not be effective vs fast-cast skills, as its duration is exactly 3 s. Practically, that's what usually happens, when you take lag and key spamming delay into account: the skill will still hit you at your target location, but your tp won't be interrupted. However, you are far safer with Silence II-III.
(5) Silence used to secure locks:
http://www.youtube.com/watch?v=8rxITv30vpI
Commentary: A difficult scenario: locking 1v3, 2 of which have good stuns/interrupts. TombMaster's Rook presents the biggest threat here. Boulder Roll has a range of 35 y, with which Silence's 15 y cannot compete. Remember, Silence is no Shield: to secure lock/tp you must Silence the opponents. What I do is faking a lock attempt. TombMaster reacts with Boulder Roll, which I dodge (you won't always be able to do it, of course, but it shouldn't matter as long as your lock does not get into cooldown mode due to being successfully interrupted). Then, with all DGs safely in range, Silence > lock.
(6) A bonus video. Extreme luring with Hedgie! Impressive, but don't try it at home:
http://www.youtube.com/watch?v=Y89PbSkeCLk
Commentary: In my opinion, this kind of maneuver requires way too much precision to have consistent practical applications, so it's just a tribute to a certain gentleman religiously playing DoW II 
