Alright, a dream pet cannot be promised to you, but there are many ways of trading up for it
1) Save up money and buy limited eggs - You can trade these eggs when they gain value so that it might give you overpays from desperate players
2) Only accepting winning or fair offers - Even if you really like a pet, think if the trade is fair or not before accepting. For example, if you like Unicorns, and someone had offered you one for your Flamingo, you should think that if you decline, you might get a better offer, and trade that for more Unicorns
3) Joining the official discord - There are many rich players on the official Adopt Me discord to trade with, which gives you a better chance of getting that pet
4) Avoid being scammed - Do not trust your items with anybody, even if you know them in real life. If you’ve been scammed, hit the report button immediately in your trading license
I hope this helped you achieve your DP!