Try harvesting in the morning if you can, before the sun gets a chance to beat down on them for any length of time. Sometimes that isn't possible with work schedules and all, so I can only tell you what I do:

After washing and separating the leaves, I leave them to soak in a sink full of cold water for about 20 minutes. Then, I gently shake the water from the leaves and put into the salad spinner. If making a salad right away, I spin the excess water off and proceed with the salad making.

If there is just too much lettuce and I have to harvest before the plants bolt, I wash and soak as above, but don't spin the leaves, just leave them in the spinner and store them in the fridge. It's amazing how long the leaves will last stored this way as opposed to storing in plastic bags or wrapped in paper towel.

Yes, afternoon is the worst possible time to pick any leaf-vegetable. I hear what you are saying about max freshness, but for lettuce it's so much better to pick in the morning, unless it's well into the evening and the leaves have returned to crispness - usually not until after 6pm. I always try to harvest the greens in the morning before leaving for work. Sometime I pull whole lettuce plants and leave the roots in a bowl of water in the sink. This morning I'm going to get chard and cauliflower greens inside shortly.

I found that my lettuce would go limp too if I let it sit for any amount of time. I'd have to pick it. Wash it in COLD water. Dry it(with paper towels) and then put it in zip lock bags, getting out as much air as possible. Then put it in the frig. It'll stay in good shape for quite a few days that way.

Another idea is to put a few rolled up damp paper towels (length-wise) in the ziploc bag with the lettuce. It helps preserve the freshness as well.

pnbrown has the right idea. the rootball might make a mess but I cut off the plant and then stick it into a wide mouth mason jar full of water. this is what we do for flowers. the stem will suck up water and the leaves will get crisp again. this seems much better then wetting down all the leaves. The cut whole head of lettuce can survive very well for days in the water but do not wait too long. or fresh change the water.

Another thing is you are growing leaf lettuce. You might be more happy with a batavia or romaine lettuce. The best two are jericho romaine or most batavia like nevada and others. cardinale is a super beautiful lettuce. I get my seeds at low cost from Fedco Seeds in Maine.

Jericho and Nevada types should stand up to the day much better and you might find you can simply cut and use even in the evening.

Personally after trying other lettuce I find the leaf lettuce to be very inferior.

Now to Aug 14 is a great time to start the fall lettuce. Fall lettuce is the best. It will stay all day in the cool weather. so get those seeds started.

Jericho is a longer grow lettuce so I started some seed a few days ago. Soon will be the best time for faster lettuce. I put Jericho at about 75 days. but they make huge heads and will supply salads for a week on one plant. just pull off the leaves. an incredible lettuce.

aug 1 to aug 15 is about right for leaf lettuce around here. but we are having a cool summer so earlier might work better this year.

Next time you are buying seeds go right to the Batavia lettuce section. if you get any lettuce make sure it is Jericho romaine.

Good Luck

Here is a link that might be useful: go to page 43 and 47 for Batavia

I forgot to pick the lettuce early and it was very hot and humid when I finally got to it. It was really limp so I let it sit in some ice water for a while and it was fine.

I pick early and take immediately in for cold water bath then through the salad spinner and into fridge. None of the varieties you mentioned will give you the crispness of iceberg - but much better lettuce flavor. I get all my lettuce seeds from www.seedsaversexchange.com as they have a great selection of lettuces for you to choose from. I had about 10 varieties this year, started indoors and planted as individual plants. Makes a great looking and tasting salad.

Thank you! Thank you!! Such wonderful information and advice. I was actually harvesting at exactly the wrong time: it's vey hot here at 3:00 p.m. Dangould, thank you so much for your detailed information on better varieties to try, and the best way to grow lettuce. I will keep this information for next year - don't think there is much seed around now, and I have already sown seedlings for August sowing. Thank you all for your time and for sharing your knowledge.
Northerner.

I store it in cold water in the fridge. Gets really crispy.

I have a huge Tupperware-style bin made by rubbermaid. My salad keeps fresh, no decline in quality, for close to a week. As long as the container is sealed the leaves will stay fresh and crisp. I also harvest first-thing in the morning. The fridge helps to remove any bitterness. The longer it's in the fridge the less bitter the salad will be. Good luck!