The hardest part of any negotiation is being tough. You want to make a demand or refuse a concession, but don't want that refusal to shut down negotiations. In those cases, having someone to play "bad cop" -- even someone who's not there -- can help.
As business blog Entrepreneur points out, having someone on your side that can stand firm even when you don't want to be confrontational can help. This can mean taking along a friend whose expertise you trust, or even using a significant other to override a particular point.
Of course you want to splurge on the XL model, but darn it if your partner just doesn't approve! Bringing along (or even referencing) someone whose role it is to play bad cop can keep negotiations open even when you're refusing their terms:
Some people are uncomfortable asserting what does and doesn't work for them point-blank. It can be helpful to have a partner. My wife is my partner. I consult her before committing to anything. The people I'm negotiating with this know this. I can tell them that I need to run something by her first, or say, "This doesn't work for Janice."
Whether or not your partner or friend would actually disapprove is immaterial. If you're both going to be present, it would help if you coordinate your intentions ahead of time.
Regardless, having someone else to take on some of your non-negotiables can help smooth the discussion. A salesperson may be willing to press you to change your mind when it's just you, but they may be less willing to press an issue when you have the support of a friend or family.
9 Ways to Negotiate a Contract Like a Boss [Entrepreneur]