As a step mum I would say they having to transport as little as possible! My SS literally moves between homes with the clothes on his back and his laptop/ipad/phone and I will even turn the clothes around (washing wise), even if I have to do it overnight, ready for him to wear it back if they are one of his ‘staple’ outfits (teen boys seem to have an obsession with 3 sets of clothes 😂).
I agree with routine, even special holidays, kids need stability. We always took the view he has siblings with mum so he always spends Christmas Eve/Day with her and Boxing Day and the day after with us for example (we have no additional siblings here).
Always support and encourage their relationship with the other parent, this becomes easier the older they get with personal devices as they can call or text the other parent whenever they please then, but facilitate contact with the other parent even if they are with you. It sends the right message on so many levels.
No matter who gets a new partner always try to work together on parenting your children first and never put your partner in the position of mum or dad.
Always take the higher ground, always put them first and allow each parent to parent on their own merits. So long as it is actively parenting.
Be flexible and talk regularly! Knowing the ins and outs of things you can support each other to support your children properly.
Honestly, it’s taken a lot of years and SS mum leaving her DH for her boyfriend (who also has two children with his ex and they co-parent really well), for her to really respect DH’s position with his son, and SS is thriving because of it now and they get on much better co-parenting!