A.14.1- The ability is written "Echo". [CompRules 2001/07/23 - 502.19a]
A.14.2- This ability means "At the beginning of your upkeep, if this permanent came under your control since the beginning of your last upkeep, sacrifice it unless you pay its mana cost." [CompRules 2001/07/23 - 502.19a]
A.14.Ruling.1- The Echo payment is required if you gained control of the creature by any means, such as putting it into play from the graveyard, taking control from another player, or having the card phase in. [D'Angelo 1999/05/01]
A.14.Ruling.2- The Echo payment is only required if you control this card at the beginning of your upkeep. [D'Angelo 1999/05/01]
A.14.Ruling.3- If you gain and lose control of the card several times before your upkeep, you still only pay the Echo cost once. [D'Angelo 1999/05/01]
A.14.Ruling.4- Effects which modify the play cost of a spell, like the Emerald Medallion or Gloom, cannot be used to modify the Echo cost. These effects only apply to the announcing of spells, and do not apply to the announcing of abilities like Echo. [D'Angelo 1998/11/25]
A.14.Ruling.5- If you manage to have it in play through an upkeep, but don't have to pay the echo (for example, if its abilities are ignored due to Humility and therefore do not trigger), then you can manage to avoid the echo payment. [D'Angelo 1999/03/03]
A.14.Ruling.6- If it enters play during your upkeep, you pay during the following upkeep. [Bethmo 1999/03/27]
A.14.Ruling.7- This ability is put on the stack (see Rule T.2) at the beginning of upkeep. If the permanent leaves play before this ability resolves, you still have to resolve it. Of course, you can just decide not to pay and there will be nothing to sacrifice. But you can also choose to pay if you really want to. [D'Angelo 1999/05/01]
A.14.Ruling.8- You decide on resolution of the triggered ability whether to pay for the Echo cost or not. [Barclay 1999/06/02] (This is not saying you decide on resolution of the spell which has Echo.)
Note- Cards used the reminder text "During your next upkeep after this permanent comes under your control, pay its mana cost or sacrifice it." Rule A.14.2 shows the more correct text.
Note- Also see Mana Cost, Rule K.18.