Chemistry - Cleaning a round-bottom flask, especially after distillation

@Permeakra suggests some good cleaning solutions. I have rearranged them (adding a few) in a suggested order from mildest to most evil. The specific thing you want to use will vary based on what you want to clean. If an initial rinse isn't good enough, you may want to let it soak for a few hours. Always take safety precautions (don't add acid washes to basic compounds, for example) and where the appropriate personal protection equipment (PPE).

• Detergent + water + "elbow grease" - there is a lab grade soap out there called "microwash" that is pretty good, and I also tell my students that things are more soluble in elbow grease than water.
• Acetone - Acetone will dissolve most organics, and it is less harmful to you and the environment than other organics.
• Dilute acid (HCl or H₂SO₄) - good for some "baked"-on stuff or for inorganic compounds. Watch out for vigorous acid-base reactions.
• Dilute nitric acid (HNO₃) - since most nitrate salts are soluble, nitric acid does a better job with resistant inorganics. It is also an oxidant and will convert resistance organics to more soluble compounds. Also watch for acid-base reactions.
• Other organic solvents - Use whatever will dissolve your crud, but use proper personal protective equipment (PPE), fume hoods, and waste disposal.
• "Base bath" (saturated KOH in isopropanol) - This solution eats a fine layer off the glass, which also removes the crud stuck to it. Rinse first with water or neutralize any acid present to avoid vigorous reaction with the bath. Immerse the flask into the bath for a few hours to overnight and it should come out clean. Base bath will dissolve silicone grease. Leave it in too long and it will come out etched. Use heavy butyl gloves.
• Concentrated HCl - more powerful than dilute acids, but more dangerous. Neutralize any bases present to avoid vigorous reaction. Use proper PPE.
• KOH + KMnO₄ + H₂O - this solution is basic (eats a fine layer of glass) and oxidative (converts many things, like organics, to a more soluble form). Neutralize any acids present and wear appropriate PPE.
• Concentrated H₂SO₄ - has the added bonus of reacting with some insoluble organics, like polyaromatics, to form soluble sulfonates. H₂SO₄ is more dangerous than HCl. Follow the same precautions, and be more careful. I would use use butyl gloves instead of nitrile or latex. It will cause serious chemical burns.
• "No-Chromix" - a non-chromium alternative to chromerge (below), no-chromix is a proprietary cleaner that is added to sufluric acid. The main ingredient is ammonium persulfate (NH₄)₂S₂O₈. You can make a reasonable knock-off by dissolving 2 g of (NH₄)₂S₂O₈ in 100 mL of water and adding to 2.5 L of sulfuric acid. Feel free to scale down, as this stuff has no shelf-life to speak of.

The following five items have dangerous reputations, and for good reasons. Use them as last resorts.

• Concentrated HNO₃ - strongly acidic and powerfully oxidizing. Dissolves many resistant things, including burned-on crud, but may ignite low molecular weight organics (like solvents). Rinse the flask first with water to remove bases, then with acetone to remove organics, and then with water to remove acetone before adding HNO₃. Use butyl gloves.
• Dilute HF (no more than 5%) - HF eats glass also, and it does so more quickly than base baths. However, the the hazards associated with HF are severe. HF burns are not like other acid burns. The HF continues to bore into you and the damage might not be evident until later. DO NOT USE UNLESS YOU HAVE THE BURN ANTIDOTE HANDY. The burn antidote is calcium gluconate, which neutralizes the acid and more important sequesters the fluoride ions. It is available as a gel or cream. You should still go overboard on PPE.
• Aqua Regia: 1 part Concentrated HNO₃ to 3 parts concentrated HCl. This is more powerful than just concentrated HNO₃, since HCl is a stronger acid. It also decomposes to form NO₂ and Cl₂, both more powerful oxidants than Concentrated HNO₃ alone. Aqua regia is famous as the only substance that can dissolve gold. It will also dissolve all manner of inorganics and organics, including silicone and hydrocarbon grease. Treat aqua regia with the respect deserved of its two components and keep it in the fume hood. Do not seal the vessels since it gives off gas. Aqua regia has no shelf life, but it remains a strong acid and will need neutralized before disposal. Note that aqua regia will not dissolve metallic rhodium, iridium, osmium, and ruthenium (all less active than gold).
• Piranha solution: H₂SO₄ + H₂O₂ - this strongly acidic strongly oxidizing solution will eat off a lot of metal, organic, and inorganic residues by oxidizing then to more soluble forms. As a strong acid oxidant, it shares the same incompatibility warnings that concentrated nitric acid has. Piranha solution is good for glassware that needs to remain calibrated (volumetrics) or uniform (NMR tubes) since it does not eat glass. Piranha solution does not have a long shelf life.
• Chromerge H₂SO₄ + K₂Cr₂O₇ - forms chromic acid, a powerful oxidant. This stuff is considered toxic waste in most jurisdictions, meaning it will be expensive to get rid of. You can store it, however, if you store it in teflon. Do not use on NMR tubes, since chromium is paramagnetic. Treat chromerge with a respect due to the sum of everything above it on this list.