I'm a full-time agent in Texas.
As others have said, lock up anything small enough to be pocketed. Kids, especially, are attracted to "neat" things and will pocket all kinds of stuff without Mom and Dad knowing.
Things to lock away...
-Jewelry
-Small collectables
-Prescription medications (that Viagra scrip will sell for $20 a pill on the street)
-Anything with concentrated value (either real or sentimental)
Things to put out of the way...
-Breakables
-Things that can trip, snag, burn, scratch, cut, or generally get in the way
De-personalize the house. As Ms. Hoppes said, potential buyers are there to imagine themselves in a new home. It's far, far easier for them to do so if your personal stuff isn't laying around. A picture here or there is fine - it makes the house feel like a home - but pictures blanketing every wall in sight make the buyers feel like they are intruding. Souvenirs, novelties, and mementos too. Make it look like an ad for a furniture store.
As tempting as it is to be there when it's shown, it's a patently bad idea. Be gone. If you're there the buyers will spend their time trying not to insult you instead of taking their time and looking at the house. With you there they will never be able to think of it as "our home". If you are worried about something being stolen, you haven't de-cluttered or secured the house enough. Go through it until you have no problem leaving it available for showing.
Being there during showings because you have something you don't want stolen later is a paranoid fallacy. Just because you are there doesn't stop people from seeing the object in question. In fact, it may make it more enticing because your body language could easily say "this object is worth a lot". The only way to keep it completely safe is for them not to see it at all. The only way to do that is lock it away or get it out of the house. If you're worried about someone coming back later while your at work and stealing something, that something shouldn't be there.
If something as polarizing as gun stuff can be moved into storage, do it. If it can't be moved out of the house, at least move it all to a single room and throw a cover over it (like you're trying to keep dust out, not hide it). That way it's concentrated in one spot and can be mentally excused by anti-gun buyers if the rest of the house is acceptable. This goes for posters, magazines, and displays, too.
If you don't have an off-site place to store your stuff, an alternative is to find a small closet somewhere in the house and put an exterior (keyed) knobset on it. It won't be Fort Knox, but it will keep prying eyes and curious hands away. Out of sight, out of mind.
Aside from that, clean and neat. If it needs painting, paint it. If it needs fixing, fix it. If it needs a good cleaning, bust out the supplies and have at it. Put in a new kitchen sink faucet and scrub the sink sparkly clean. Pop the sixty or seventy bucks for that new bathroom faucet and shower diverter. Spend a couple of hours scrubbing the toilet and tub enclosure. Wipe down all the switch and outlet faceplates (replace if necessary, they are pathetically cheap). Scrub all the guck off your doors that accumulates around the knobs and pulls. Lemon oil any exposed wood. Etc, etc, etc.
And for heaven's sake don't try to impress potential buyers by putting scented somethings all around the house. Heavy scents turn people off and make people wonder what you're trying to cover up. Your house should smell like nothing. At most, there should only be the slightest whiff of something neutral (vanilla or citrus). If you want to put in a few cookies to bake, fine, but do it hours ahead of time, not just as they walk in.
I can't recommend enough getting a pre-sale inspection. Homeowners become blinded by familiarity to their home's faults. An inspection will show you the things that will eventually need to be addressed anyway. Once you get everything taken care of, you can have the inspector come back in. Use both inspections plus any repair invoices to show the potential buyer how much better your house is than the one down the street where the seller hasn't taken the time up front to make things right. Gives the buyer a sense of security about your house (removes potential objections).
Brad