[Tinn]

1. Bust-increasing herbs: To increase bust size and the amount of produced milk a cowgirl can store without feeling uncomfortable. I favour herbs over potions and magical doodads as transformative items because, if you think about it, cows eating plants sort of works.
2. Milking stool: To allow cowgirls to be milked without machines. It might not be as efficient as the mechanical option (especially at lower skill levels), but it would be cheaper to run and maintain, and the more personal touch could be better for morale, relationships and/or sexy text descriptions.
3. Cow-print Bikini: Because Muhznit was onto something: It's a classic cowgirl fashion choice, fits a variety of body shapes, and is cheap in terms of materials. Besides, in a lot of places it's the smallest amount of clothing a girl can legally wear in a public, which is likely to be of interest to at least some players.
4. Milk urns: Because it always struck me as weird that so many games with milking allowed the user to create containers for milk out of thin air as part of the milking process. Milk urns are an important and realistic part of farm logistics! Also, having to keep a sufficient supply of empy urns could add an extra element of strategy to gameplay.
5. Milkmaid outfit: Because it's too obvious to be left out. Wearing one should probably increase a cowgirl's ability to milk other cowgirls, her rate of milk production, or both.

Co-incidentally, I do think it would add something if clothing could still provide bonuses when a cowgirl's skills are maxed out.

Reading your blog, Shinigamis Past, I see you're trying to get out of the indentured servitude/slavery concept. I've already posted a couple of ideas on that front, but here's another couple:
1. Have the farm run as a sort of shared commune, where the cowgirls partners, instead of property. This would explain why they don't get wages, encourage the player to think of them as friends, and would open up the possibility of playing as a cowgirl.
2. Have the cowgirls be volunteers working in exchange for basic food and shelter. This would work, but is pretty unrealistic, and would open the game up to unpleasant interpretations, such as being about homeless girls bribed and blackmailed into working for less than minimum wage.
3. Actually pay the cowgirls a nominal wage. The amount needn't be much, as long as cowgirls contribute more revenue to the farm in a day then they get paid in the same amount of time. Realistically, this is how businesses operate in the real world - and it's not even hard to balance: Just add one cowgirlmonth's wages to the amount of money you want the player to earn from a single basic cowgirl's worth of milk per month.
Heck, you could even make wages paid have an effect on morale: Not paying on time could reduce it, wheras giving bonuses could gived it a boost - though there'd have to be a limit on frequency to prevent that from getting too grindable.

Anyway, if you really want to avoid connotations of slavery while keeping the 'pay money for more cowgirls' mechanic, you could make the 'cost' of a new girl be a sort of bride-price paid to their families for the priviledge of living with their daughter. This would make the player's herd more like a family, albeit one with a noteworthy gender imbalance.